Optical displacement sensing device with reduced sensitivity to misalignment

ABSTRACT

An optical displacement sensing device is provided for determining the relative displacement of a diffraction grating scale along a measuring axis. The grating may be reflective and the grating pitch may be less than the wavelength of the light of the displacement sensing device. The sensing device includes a split light beam input portion for inputting two split light beams along respective light paths, and light beam directing elements for directing the two split beams to converge proximate to a first zone on the scale grating to give rise to two diffracted beams which diverge proximate to the first zone. The sensing device further includes retroreflector elements for receiving and retroreflecting the two diffracted beams to converge proximate to a second zone on the scale grating to give rise to two later-diffracted light beams which diverge and are directed to a shared zone. An optical detector detects at least one illumination characteristic arising from the shared zone, thus sensing displacement of the grating scale along the measuring axis.

FIELD OF THE INVENTION

[0001] This invention relates to a sensing device and, moreparticularly, to an optical displacement sensing device that utilizesthe interference of light beams for detecting relative displacements ofa diffraction grating with reduced sensitivity to misalignments.

BACKGROUND OF THE INVENTION

[0002] An optical displacement sensing device, or optical encoder, isdescribed that can overcome several of the most significant problemsfaced by designers of these types of devices for practical highprecision measurement. If an optical displacement sensing device is tobe used to make high precision measurements of a grating surfacedisplacement, e.g., sub-micron resolution and accuracy, it musteffectively eliminate or attenuate any distortions of such measurementsat a very high level. Current optical displacement sensing devices areincapable of economically and practically eliminating or significantlyreducing the effects of certain distortions or parameter drifts fromtheir measurements at the desired levels of accuracy and resolution.Among the problems frequently encountered while using such devices arethose arising from a change in wavelength of the light source used forperforming measurements, such as associated changes in diffractionangles, changed lights paths, and changes in the number of wavelengthsoccurring in two light paths of differing length—which affects theirrelative phase and interference pattern.

[0003] Another problem is associated with very small grating periods. Toachieve high measurement resolution, it is desirable to use a scalegrating with as short a grating period, d, as possible. The lower limitis set by the wavelength λ of the light source, according to the formulad>λ/2. However, unless special design measures are taken, an encoderusing such a short scale grating period is too difficult to align withsufficient yaw accuracy, requiring expensive equipment or excessive timeand care during installation. Yaw misalignment is a rotation of theoptical readhead relative to the grating, in a plane parallel to thegrating.

[0004] With a yaw misalignment, the desired output beams arising fromthe scale are no longer parallel and related “distortion” interferencefringes are created. If the distortion interference fringe period issmall relative to the diameter of the detector area illuminated by theinterfering beams, the desired modulation of the signal due to gratingmotion will be significantly attenuated because several distortionfringe periods will fit within the detector area, and the detectorsignal will tend toward the constant average intensity of thesedistortion fringes. To avoid this effect when using practical types ofoptical detectors and a grating period that approaches the previouslydiscussed λ/2 limit, the yaw misalignment must be smaller thanapproximately 0.1 milliradians, in the absence of special designmeasures. Such alignment requirements are impractical for many users andapplications. It is known to incorporate retroreflectors in the lightpaths of optical encoders in order to overcome such yaw problems, asshown in U.S. Pat. Nos. 5,079,418 to Dieter and 4,930,895 to Nishimura,each of which is incorporated herein by reference, in its entirety.However, such arrangements of retroreflectors have not simultaneouslyconsidered compact and economical optical readhead design and packaging,the versatility to work with grating periods both greater and lesserthan the wavelength of the light source, and measurement insensitivityto various parameter drifts—including misalignments other than yaw. Allof these design factors must be considered simultaneously, and theproper tradeoffs chosen, in order to robustly achieve currently desiredmeasurement resolutions and accuracies.

[0005] In particular, parameter drifts including dynamic positionmisalignments are important error sources in an optical displacementsensing device. Herein, the term dynamic misalignment or drift means thechange in an alignment component or parameter that occurs between onedisplacement position and another displacement position, or at the samedisplacement position over a period of time, for any reason. Among therange of possible dynamic position misalignments and drift that areintroduced in practical applications are changes in the gap between thereadhead and grating, pitch (rotation about an axis parallel to thegrating and normal to the measuring axis), yaw, roll (rotation about anaxis parallel to the measuring axis), and drift in the wavelength of alight source. U.S. Pat. No. 5,146,085 to Ishizuka, incorporated hereinby reference, in its entirety, and the '895 patent, previouslyincorporated, both disclose optical readhead configurations which arerelatively insensitive to errors associated with pitch. However, theseconfigurations are not versatile and robust enough when considering theplacement of retroreflectors in combination with consideration of theother design factors noted for simultaneous consideration above. Thus,error sensitivities associated with the relative pitch of the readheadand grating remain as some of the most difficult error sources to reducein practical high resolution encoder design and application.

[0006] Furthermore, the '085 and '895 configurations may introduceproblems created by the reflection of a light beam into the light sourceused for the generation of the light beam, leading to instability in thewavelength of the light source. Also, these configurations requirepolarizers, which attenuate the light available to the optical detectorand which may limit or prohibit the configuration for use with certaindetectors and/or impose relatively higher system power requirements,which may complicate or limit their use in certain applications.

[0007] The present invention is directed to optical readheadconfigurations which are suitable for compact and economical design andpackaging, versatile enough to apply with grating periods both greaterand lesser than the wavelength of the light source, and which aresubstantially simultaneously insensitive to various parameter driftsincluding at least dynamic yaw and pitch misalignments. Someconfigurations also avoid or limit attenuation of the light available tothe detector and/or avoid reflection of light beams into the lightsource.

SUMMARY OF THE INVENTION

[0008] In accordance with this invention, an optical displacementsensing device or optical encoder readhead is provided for determiningthe relative displacement of a diffraction grating scale along ameasuring axis. The grating may be reflective and the grating pitch maybe less than the wavelength of the light of the encoder readhead. Thesensing device includes a split light beam input portion for inputtingtwo split light beams along respective light paths, light beam directingelements for directing the two split beams to converge proximate to afirst zone on the scale grating to give rise to two diffracted beamswhich diverge proximate to the first zone, retroreflector elements forreceiving and retroreflecting the two diffracted beams to convergeproximate to a second zone on the scale grating to give rise to twolater-diffracted light beams which diverge and are directed to a sharedzone, and an optical detector. The optical detector detects at least oneillumination characteristic arising from the shared zone, thus sensingdisplacement of the grating scale along a measuring axis.

[0009] In accordance with an aspect of the invention, by directing thetwo split beams to converge proximate to the first zone, the devicesensitivity to dynamic pitch misalignments can be reduced.

[0010] In accordance with a further aspect of the invention, byretroreflecting the two diffracted beams to converge proximate to thesecond zone, yaw sensitivity is substantially eliminated while thedevice sensitivity to dynamic pitch misalignments is further reduced.

[0011] In accordance with a further aspect of the invention, the sensingdevice configuration may be aligned with either one vertical planeperpendicular to the surface of the scale and parallel to the measuringaxis, or with two different inclined planes inclined away from each andparallel to the measuring axis. Various inclined configurations canprevent undesirable mixing of various light beams, while at the sametime facilitating compact design and packaging of the device.

[0012] In accordance with a further aspect of the invention, the lightpaths which diverge proximate to the first zone each receive adiffracted light beam arising from only one of the split beams, avoidingthe need for polarizers in the encoder readhead.

[0013] In accordance with a further aspect of the invention, the lightpaths which diverge proximate to the second zone each receive alater-diffracted light beam arising from only one of the split beams,further avoiding the need for polarizers in the encoder readhead.

[0014] In accordance with a further aspect of the invention, the sensingdevice configuration may be arranged such that the split light beamsnominally impinge on the first zone within a range of angles thatattenuates the sensitivity of the device to certain errors associatedwith at least dynamic pitch changes and wavelength drift, while at thesame time facilitating compact design and packaging of the device.

[0015] In accordance with another aspect of the invention, the lightbeam directing elements comprise opposing surfaces of a block oftransparent material, providing a compact, economical and robust way tofabricate and precisely position the elements.

[0016] In accordance with another aspect of the invention, theretroreflector elements comprise respective corners of a block oftransparent material, providing a compact, economical and robust way tofabricate and precisely position the retroreflector elements.

[0017] In accordance with another aspect of the invention, a readheadgrating produces the two split beams and the sensing deviceconfiguration may be arranged such that the readhead grating andretroreflectors are in a configuration that attenuates the sensitivityof the device to certain errors associated with at least dynamic pitchchanges and wavelength drift.

[0018] In accordance with another aspect of the invention, abeamsplitter produces the two split beams and the sensing deviceconfiguration may be arranged such that the beamsplitter andretroreflectors are in a configuration that attenuates the sensitivityof the device to certain errors associated with at least dynamic pitchchanges and wavelength drift.

[0019] In accordance with a further aspect of the invention, in variousembodiments a polarizing configuration may be used for the opticaldetector and in various other embodiments a compact optical array may beused for the optical detector.

[0020] Hence, the invention overcomes the disadvantages of prior artoptical displacement sensing devices with a compact, economical andversatile configuration applicable to grating periods both greater andlesser than the wavelength of the light used, and which is substantiallysimultaneously insensitive to various parameter drifts including atleast dynamic yaw and pitch misalignments.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The foregoing aspects and other attendant advantages of thisinvention will become more readily appreciated as the same become betterunderstood by reference to the following detailed description, whentaken in conjunction with the accompanying drawings, wherein:

[0022]FIG. 1 is a diagram showing a side view of light beam paths in afirst prior art optical displacement sensing device;

[0023]FIG. 2 is a diagram showing a schematic side view of a secondprior art optical displacement sensing device;

[0024]FIG. 3 is a diagram showing a schematic side view of a third priorart optical displacement sensing device;

[0025]FIG. 4 is a three-dimensional conceptual view of an opticaldisplacement sensing device configuration in accordance with a firstexemplary embodiment of the invention;

[0026]FIG. 5A is a three-dimensional conceptual view of an opticaldisplacement sensing device in accordance with a second exemplaryembodiment of the invention;

[0027]FIG. 5B is a side view of the optical displacement sensing deviceshown in FIG. 5A;

[0028]FIG. 5C is a top view of the optical displacement sensing deviceshown in FIG. 5A;

[0029]FIG. 5D is an end view of the optical displacement sensing deviceshown in FIG. 5A;

[0030]FIG. 6 is a three-dimensional schematic view clarifying thegeometric components and symbols used for describing light pathdirections herein;

[0031]FIG. 7 is a three-dimensional schematic view showing a genericinput light ray direction and the resulting conical distribution ofdiffracted light rays from a diffraction grating, using the geometriccomponents and symbols shown in FIG. 6;

[0032]FIG. 8 is an error table showing the errors associated withvarious initial and dynamic misalignments and drifts for a prior artoptical displacement sensing device configuration corresponding to FIG.1;

[0033]FIG. 9 is an error table showing the errors associated withvarious initial and dynamic misalignments and drifts for an opticaldisplacement sensing device configuration corresponding to FIGS. 5A-5Dand FIGS. 12A-12D;

[0034]FIG. 10A is a three-dimensional view clarifying the operation of afirst readhead grating configuration usable in various exemplaryembodiments according to this invention;

[0035]FIG. 10B is a three-dimensional view clarifying the operation of asecond readhead grating configuration usable in various exemplaryembodiments according to this invention;

[0036]FIG. 10C is a three-dimensional view clarifying the operation of athird readhead grating configuration usable in various exemplaryembodiments according to this invention;

[0037]FIG. 10D is a three-dimensional view clarifying the operation of afourth readhead grating configuration usable in various exemplaryembodiments according to this invention;

[0038]FIG. 10E is a three-dimensional view clarifying the operation of afifth readhead grating configuration usable in various exemplaryembodiments according to this invention;

[0039]FIG. 11A is a three-dimensional view clarifying the operation of afirst beamsplitter configuration usable in various exemplary embodimentsaccording to this invention;

[0040]FIG. 11B is a three-dimensional view clarifying the operation of asecond beamsplitter configuration usable in various exemplaryembodiments according to this invention;

[0041]FIG. 11C is a three-dimensional view clarifying the operation of athird beamsplitter configuration usable in various exemplary embodimentsaccording to this invention;

[0042]FIG. 11D is a three-dimensional view clarifying the operation of afourth beamsplitter configuration usable in various exemplaryembodiments according to this invention;

[0043]FIG. 12A is a three-dimensional conceptual view of an opticaldisplacement sensing device in accordance with a third exemplaryembodiment of the invention;

[0044]FIG. 12B is a side view of the optical displacement sensing deviceshown in FIG. 12A, including a portion of an exemplary readhead housing;

[0045]FIG. 12C is a top view of the optical displacement sensing deviceshown in FIG. 12A;

[0046]FIG. 12D is an end view of the optical displacement sensing deviceshown in FIG. 12A;

[0047]FIG. 13 is a graph showing error sensitivity data for dynamic gapmisalignment and wavelength change at various incident beam angles in anoptical displacement sensing device in accordance with this invention;

[0048]FIG. 14 is a table showing illumination spot length data and dataindicative of readhead size, for various incident beam angles in anoptical displacement sensing device in accordance with this invention;

[0049]FIG. 15 is a graph showing error sensitivity data for dynamic gapmisalignment at various incident beam angles and at various inclinedplane angles, using a 635 nm wavelength in an optical displacementsensing device in accordance with this invention;

[0050]FIG. 16 is a graph showing error sensitivity data for dynamic gapmisalignment at various incident beam angles and at various inclinedplane angles, using a 405 nm wavelength in an optical displacementsensing device in accordance with this invention;

[0051]FIG. 17 is a graph showing error sensitivity data for wavelengthdrift at various incident beam angles and at various inclined planeangles, using a 635 nm wavelength in an optical displacement sensingdevice in accordance with this invention;

[0052]FIG. 18 is a graph showing error sensitivity data for wavelengthdrift at various incident beam angles and at various inclined planeangles, using a 405 nm wavelength in an optical displacement sensingdevice in accordance with this invention;

[0053]FIG. 19 is a three-dimensional conceptual view of an opticaldisplacement sensing device in accordance with a fourth exemplaryembodiment according to the invention;

[0054]FIG. 20A is a conceptual side view of an optical displacementsensing device in accordance with a fifth exemplary embodiment accordingto the invention;

[0055]FIG. 20B is a top view of the optical displacement sensing deviceshown in FIG. 20A;

[0056]FIG. 20C is an end view of the optical displacement sensing deviceshown in FIG. 20A;

[0057]FIG. 21 is a schematic side view of an optical detector includinga polarizer configuration usable in various exemplary embodimentsaccording to the invention;

[0058]FIG. 22A is a conceptual side view of an optical displacementsensing device in accordance with a sixth exemplary embodiment accordingto the invention;

[0059]FIG. 22B is a top view of the optical displacement sensing deviceshown in FIG. 22A;

[0060]FIG. 22C is an end view of the optical displacement sensing deviceshown in FIG. 22A;

[0061]FIG. 23 is a conceptual side view of an optical displacementsensing device in accordance with a seventh exemplary embodimentaccording to the invention;

[0062]FIG. 24 is a conceptual side view of an optical displacementsensing device in accordance with an eighth exemplary embodimentaccording to the invention;

[0063]FIG. 25 is a three-dimensional view showing a first exemplarytransparent block configuration usable in various exemplary embodimentsaccording to the invention;

[0064]FIG. 26A is a three-dimensional view showing a second exemplarytransparent block configuration usable in various exemplary embodimentsaccording to the invention;

[0065]FIG. 26B is a side view of the transparent block configurationshown in FIG. 26A;

[0066]FIG. 26C is an end view of the transparent block configurationshown in FIG. 26A;

[0067]FIG. 27A is a three-dimensional view showing a third exemplarytransparent block configuration usable in various exemplary embodimentsaccording to the invention;

[0068]FIG. 27B is a side view of the transparent block configurationshown in FIG. 27A;

[0069]FIG. 27C is an end view of the transparent block configurationshown in FIG. 27A; and

[0070]FIG. 28 is a flow diagram for a method of using an opticaldisplacement sensing device in accordance with various exemplaryembodiments of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0071] The invention provides a configuration for a high resolutiondisplacement sensing device or optical encoder that is compact,relatively economical to construct, and which is substantiallysimultaneously insensitive to various parameter drifts including atleast dynamic yaw and pitch misalignments. The configuration isversatile enough to be applicable to grating periods both greater andlesser than the wavelength of the light used.

[0072]FIG. 1 is a diagram showing a side view of light beam paths in afirst prior art optical displacement sensing device. The separation oflight rays in the diagram are exaggerated for clarity. A relativedisplacement axis and/or measuring axis direction 101 is indicated. Abeam which is the source of input rays 100 a and 100 b is transmittedtoward readhead grating 102. Readhead grating 102 is a transmissivediffraction grating. A ray tracing process is started at location 110which is a wavefront at which input rays 100 a and 100 b have the sameoptical phase. The starting points of the rays on the wavefront arechosen so that, when traced through the system, both rays arrive at thesame output detection point 106.

[0073] Light ray 100 a is diffracted at deflection point X₄ and producesa light ray N₄, a chosen diffraction order. Light ray N₄ is diffractedfrom scale surface 104 at reflection point X₅ and forms diffracted rayN₅. Diffracted ray N₅ is transmitted in the gap between scale surface104 and readhead grating 102. Light ray N₅ is subsequently transmittedthrough readhead grating 102 at transmission point X₆ and produces azero order transmitted light ray N₆ which is transmitted to outputdetection point 106. In a similar fashion, input ray 100 b istransmitted through readhead grating 102 at transmission point X₁ andproduces a zero order transmitted light ray N₁, which is subsequentlytransmitted through the gap between readhead grating 102 and scalesurface 104. Light ray N₁ is diffracted from scale surface 104 atreflection point X₂, and produces a light ray N₂, a chosen diffractionorder. Light ray N₂ is diffracted from readhead grating 102 atdeflection point X₃ and produces light ray N₃, a chosen diffractionorder. Light ray N₃ is subsequently transmitted in a direction parallelor nearly parallel to transmitted light ray N₆ to output detection point106. It should be appreciated that the deflection points X₁ and X₄ maybe the same point and the deflection points X₃ and X₆ may be the samepoint.

[0074] The prior art configuration shown in FIG. 1 is intended todemonstrate the fundamental problems that exist in the absence ofmodifications to overcome the effect of dynamic misalignments. Oneprimary objective of the claimed invention is to provide accurate outputbeam signals and related measurements while rejecting variouscombinations of static and dynamic misalignments. Among the potentialsources of dynamic misalignments to be overcome by the invention aremisalignments during the travel of the displacement sensing devicerelative to a target object due to bearing imperfections, and due tomisalignments arising from warping and waviness of the surface of thegrating. Warping and waviness, for example, result from thicknessvariations in the measuring scale and/or a bonding layer attaching themeasuring scale to the target object or an intermediate mounting member,as well as from the measuring scale conforming to surface distortions ofa mounting member. FIG. 1 shows scale surface 104 elevated from aninitial or ideal surface parallel to readhead grating 102 and ormeasuring axis direction 101 by an angle 108. Angle 108 represents adynamic pitch misalignment of the scale surface 104 relative to anoptical displacement sensing device.

[0075] The dynamic pitch misalignment of angle 108, for example due todistortion, thermal deformation, or displacement creating a change inelevation between reflection points X₂ and X₅, would change thepositions of the points X₁-X₆, where the rays intersect with thegratings. This change of where the rays intersect the gratings creates achange in the phase difference between the two light rays, N₃ and N₆,that will be indistinguishable from any change in phase difference dueto the intended displacement of the scale surface 104 along measuringaxis direction 101, resulting in measurement errors. The errors will beproportional to the magnitude of the dynamic misalignment encounteredbetween successive measurements of the displacement of the scale surface104. An alternative configuration is required to overcome suchinaccuracies that will be produced from dynamic misalignments that occurwhen making measurements with a device having the prior artconfiguration shown in FIG. 1.

[0076]FIG. 2 is a diagram showing a schematic side view of a secondprior art optical displacement sensing device disclosed in theincorporated '085 patent. In FIG. 2, the reference numeral 1 designatesa light source comprising a laser diode, the reference numeral 2 denotesa collimator lens, the reference numeral 9 designates a polarizing beamsplitter, the reference numeral 5 denotes a diffraction grating having apitch P formed on a linear scale or a rotary scale, the referencenumerals 61 and 62 designate mirrors, the reference numerals 7A and 7Bdenote quarter-wavelength plates, the reference numeral 6 designates anon-polarizing beam splitter, the reference numerals 71 and 72 denotepolarizing elements (such as polarizing plates or polarizing beamsplitters), and the reference numerals 81 and 82 designate lightreceiving elements. A laser beam of wavelength λemitted from the lightsource 1 is collimated by the collimator lens 2, and the parallel lightbeam is caused to enter the polarizing beam splitter 9, whereby it isdivided into two light beams R1 and R2 whose polarization azimuths areorthogonal to each other. The light beam R1 is an S-polarized lightreflected by the polarizing beam splitter 9, and the light beam R2 is aP-polarized light transmitted through the polarizing beam splitter 9.The light beam R1 travels along an optical path L1 formed via the mirror61, and the light beam R2 travels along an optical path L2 formed viathe mirror 62. The light beams R1 and R2 pass through the quarterwavelength plates 7A and 7B, whereafter they are incident on a point P1on the diffraction grating 5 at an angle of incidence θo=θb=sin⁻¹(λ/2P), and light beams R1 and R2 are obtained by being reflected anddiffracted by the diffraction grating 5. The +1st-order diffracted light(R1+) of the light beam R1 and the −1st-order diffracted light (R2−) ofthe light beam R2 travel toward the original optical paths L1 and L2,respectively, through the quarter wavelength plates 7A and 7B. The+1st-order diffracted light traveling reversely along an optical path L1and the −1st-order diffracted light traveling reversely along an opticalpath L2 are reflected by the mirrors 61 and 62, respectively, and aredirected to the polarizing beam splitter 9, and are again superposed oneupon the other by the polarizing beam splitter 9. The +1st-orderdiffracted light is made into a P-polarized light by the action of thequarter wavelength plate 7B and the −1st-order diffracted light is madeinto an S-polarized light by the action of the quarter wavelength plate7A and therefore, these light beams emerge from the polarizing beamsplitter 9 while overlapping with each other, without any loss. Theoverlapping two light beams pass through the quarter wavelength plate 53and become circularly polarized lights.

[0077] After this the light beam is divided into two light beams equalin quantity of light by the non-polarizing beam splitter 6. Only aparticular polarized component is separated and taken out from one ofthe two light beams by the use of the polarizing element 72 and iscaused to enter the light receiving element 82, and only a particularpolarized component is separated and taken out from the other light beamby the use of the polarizing element 71 and is caused to enter the lightreceiving element 81. Periodic signals are output from the lightreceiving elements 82 and 81, respectively in conformity with thedisplacement of the scale. The periodic signals are output in“quadrature”, according to methods known to one skilled in the art. Thestructure and operation of the configuration shown in FIG. 2 aredescribed in further detail in the incorporated '085 patent.

[0078] The prior art configuration shown in FIG. 2 substantiallyeliminates the dynamic pitch misalignment sensitivity previouslydiscussed with respect to FIG. 1. However, the configuration hasundesirable limitations. The configuration includes no means forreducing yaw sensitivity. With yaw misalignment, the output beamsreturning from the scale are no longer parallel and the signals providedby the optical detection scheme will produce large errors at small yawmisalignments. Thus, the configuration is not robust and requires alevel of care in installation and use that is not desirable. This isparticularly true when the pitch P of diffraction grating 5 is madesmall, as previously discussed. In addition, the zero order beams“crossover” to the opposite optical path by reflection at the point P1.Thus, “crossover beams” are undesirably combined with the desireddiffracted beams described above. Light from the crossover beams must beremoved from the respective optical paths by the effects of polarizingbeam splitter 9. Thus, the configuration must use polarizers, whichimpose unwanted encoder readhead fabrication and/or assembly constraintsin some situations. Optical energy is also wasted. Furthermore, thedisclosed configuration lies in a plane which is normal to the scale.Thus, a portion of the reflected and diffracted light may be returned tothe light source, causing instability in the light source. Furthermore,the wavelength λ and the grating pitch P completely determine theincident angle, which imposes unwanted encoder readhead packaging andsize constraints in some situations. An alternative configuration isrequired to overcome such undesirable limitations.

[0079]FIG. 3 is a diagram showing a schematic side view of a third priorart optical displacement sensing device that is disclosed in theincorporated '895 patent. In FIG. 3, the reference numeral 1 designatesa semiconductor laser, the reference numeral 2 denotes a collimatorlens, the reference numeral 9 designates a polarizing beam splitter, thereference numeral 5 denotes an optical scale having a diffractiongrating having a pitch P, the reference numerals 61 and 62 designatereflecting mirrors, the reference numerals 8A and 8B denotequarter-wavelength plates, the reference numeral 6 designates a beamsplitter, the reference numerals 71 and 72 denote polarizing plateswhose polarization axes form an angle of 45° with each other. Thereference numerals 81 and 82 designate light receiving elements whichphotoelectrically convert interference fringes. The reference numeral 11denotes an index distribution type stick lens carrying a reflecting film12, which together constitute a reflecting element 20 that returns lightsubstantially along the optical path by which it entered the reflectingelement 20. A coherent light beam from the semiconductor laser 1 iscollimated by the collimator lens 2 and enters the polarizing beamsplitter 9, whereby it is divided into transmitted and reflected lightbeams whose polarizations are orthogonal to each other. The transmittedand reflected light beams are made into circularly polarized lightsthrough the quarter wavelength plates 8A and 8B, respectively, and arereflected by the reflecting mirrors 62 and 61, and are caused toobliquely enter the optical scale 5 at a point P1 so that the mth-orderdiffracted light from the optical scale 5 emerges substantiallyperpendicularly from the diffraction grating surface of the opticalscale 5. That is, each light beam is caused to enter the optical scale 5so that θ_(m)≈sin⁻¹(mλ/P), where P is the grating pitch, λ is thewavelength of the light from the semiconductor laser 1, m is an integer,and θ_(m) is the incident angle, from the light beam to theperpendicular to the diffraction grating surface. The diffracted lightsemerging substantially perpendicularly from the diffraction gratingsurface form a common optical path and enter the reflecting element 20where they are reflected to return along the original optical path, arereflected by the reflecting mirrors 61 and 62, are transmitted throughthe quarter wavelength plates 8A and 8B and again enter the polarizingbeam splitter 9. The diffracted lights emerge from the polarizing beamsplitter 9 superimposed and are made into circularly polarized lightsopposite in direction to each other through the quarter wavelength plate53.

[0080] After this the superimposed lights are divided into two lightbeams by the beam splitter 6, and are made into linearly polarizedlights through the polarizing plates 72 and 71 and then enter the lightreceiving elements 82 and 81, respectively. Periodic signals are outputfrom the light receiving elements 82 and 81, respectively in conformitywith the displacement of the scale. The periodic signals are output in“quadrature”, according to methods known to one skilled in the art.Because the received light is twice diffracted by the optical scale 5 asmth-order light, once before entering the reflecting element 20 and onceafter, when the grating moves by one pitch increment the periodicsignals undergo 4 m cycles. The structure and operation of theconfiguration shown in FIG. 3 are described in further detail in theincorporated '895 patent.

[0081] The prior art configuration shown in FIG. 3 substantiallyeliminates the dynamic pitch misalignment sensitivity previouslydiscussed with respect to FIG. 1. Furthermore, the configurationsubstantially eliminates yaw sensitivity through the effects of thereflecting element 20, which is one type of retroreflector.Nevertheless, the configuration has all the other undesirablelimitations previously discussed with respect to FIG. 2, as well as oneadditional major limitation. The additional limitation is that theconfiguration will not operate with a grating pitch P that is less thanthe wavelength λ of the light. That is, there is no solution to therequired equation θ_(m) sin⁻¹(mλ/P), in this situation. In addition,even when the configuration is operable, the zero order beams and“crossover” to the opposite optical path by reflection at the point P1,and the diffracted beams are otherwise mixed in the common path to andfrom the reflecting element 20. Thus, “crossover beams” are undesirablycombined with the desired diffracted beams. Light from the crossoverbeams must be removed from the respective optical paths by the effectsof polarizing beam splitter 9. Thus, the configuration must usepolarizers, which impose unwanted encoder readhead fabrication and/orassembly constraints in some situations. Optical energy is also wasted.Furthermore, the disclosed configuration lies in a plane which is normalto the scale. Thus, a portion of the reflected and diffracted light maybe returned to the light source, causing instability in the lightsource. Furthermore, the wavelength λ and the grating pitch P completelydetermine the incident angle, which imposes unwanted encoder readheadpackaging and size constraints in some situations. An alternativeconfiguration is required to overcome such undesirable limitations.

[0082] In the following discussions of various exemplary embodimentsaccording to the principles of this invention, only operable light beamsand/or light paths are shown, as necessary to explain and clarify theinvention. However, it should be appreciated that various beam splittingelements and gratings shown and discussed below will give rise tovarious other split beams and/or diffracted light beam orders which are“lost” from the various operable light path configurations according tothis invention. Such “lost light” paths and/or beams are generally notshown or discussed except herein, except with regard to their potentialfor light source wavelength disturbance or crossover light in aparticular embodiment. Accordingly, it should be appreciated that forsimplicity and clarity terms such as “light beam” or “light path” areused herein to refer to the operable light beams or operable light pathsthat contribute light that is eventually detected in a shared zoneaccording to the principles of this invention, unless otherwiseindicated. The various other “lost light” paths and/or beams which maybe present in various embodiments according to this invention will beapparent to one of ordinary skill in the art. It is to be understoodthat in addition to the operable light paths and/or beams described andclaimed below, such lost light paths and/or beams are present in variousembodiments according to this invention regardless of whether or notthey are explicitly indicated.

[0083]FIG. 4 is a three-dimensional conceptual view of an opticaldisplacement sensing device configuration in accordance with a firstexemplary embodiment of this invention. This first exemplary embodimentis discussed as a generic embodiment, which may include the specificexemplary elements shown in FIG. 4. It should be appreciated that thegeneric portions of the following discussion are indicative of largenumber of configurations which vary in their combination of elements anddimensions from the specific elements and dimensions indicated in FIG.4. Thus, FIG. 4 should not be interpreted as limiting with regard to themeaning and intent of the generic portions of the following discussion.

[0084]FIG. 4 shows an encoder readhead configuration 400. Thisembodiment is a practical configuration that is substantiallyinsensitive to various misalignments and dynamic misalignments, yetoffers substantial flexibility to locating and fabricating andassembling various optical components in a compact and economical way inan encoder readhead. The encoder readhead includes a split light beaminput portion 410, light beam directing elements 420 and 421,retroreflectors 440 and 441, a shared zone 450 and an optical detector460 having one or more power and/or signal connections 461. Theconfiguration also includes a scale grating 430, a first zone 431 on thegrating scale, and a second zone 432 on the grating scale.

[0085] In the exemplary embodiment shown in FIG. 4, the split light beaminput portion 410 includes a light source 412 and a beam splittingelement 415. The light source 412 emits coherent radiation and invarious exemplary embodiments, the light source 412 includes acollimating element and emits collimated light. In various exemplaryembodiments according to this invention, the light source 412 may be afiber optic element which conveys light from a remote light source, or alight emitting diode or a laser diode included in the encoder readhead.In various exemplary embodiments, a laser diode is used due to thelonger coherence length of the emitted light. In various other exemplaryembodiments, a vertical cavity surface emitting laser diode is used dueto the greater temperature stability of the emitted wavelength. The beamsplitting element 415 receives a light beam 401 from the light source412 and produces two split light beams 401 a and 401 b along respectivelight paths according to known optical principles and inputs the beamsto the remainder of the encoder readhead configuration 400.

[0086] It should be appreciated that in the exemplary embodiment shownin FIG. 4, the beam splitting element 415 is conceptually illustrated asa readhead grating having a first portion impinged upon by the lightbeam 401. However, in this and various other exemplary embodimentsherein, the beam splitting element 415 impinged upon by the light beam401 is more generally intended to represent any now known or laterdeveloped light beam splitting element or combination of elements, suchas a suitably arranged “half-silvered” beamsplitter or a polarizingbeamsplitter or portions thereof, or portions of a grating or multiplegratings or the like, including any of the configurations discussedbelow and shown in 10A-10E or 11A-11D, which is operable to producesplit light beams according to the principles of this invention.

[0087] Furthermore, it should be appreciated that in various exemplaryembodiments, according to known optical miniaturization and assemblytechniques the split light beam input portion 410 may incorporate alight source, collimation, beam splitting, and polarizing functions,into a single integrated optical element or assembly, such that thelight source 412 and the beam splitting element 415 are difficult orimpossible to distinguish as separate elements.

[0088] The split light beams 401 a and 401 b are directed alongrespective light paths by the light beam directing elements 420 and 421such that the split light beams 401 a and 401 b converge proximate tothe first zone 431 on the grating scale. In the embodiment shown in FIG.4, and various other embodiments shown herein, the light beam directingelements 420 and 421 are each represented as exemplary plane mirrorsthat reflect the split light beams 401 a and 401 b, respectively, oncefrom a first portion of each plane mirror. However, more generally, eachof the light beam directing elements 420 and 421 impinged upon by thesplit light beams 401 a and 401 b may be any now known or laterdeveloped light beam directing element or combination of elements, suchas suitably arranged mirrors and/or portions of a grating or multiplegratings or the like, that are operable through one or more reflectionsor deflections to direct the light beams 401 a and 401 b alongrespective light paths to converge proximate to the first zone 431.

[0089] The first zone 431 is a nominal zone on the nominal plane of thescale grating 430 that has a dimension along the measuring axisdirection 101 that bounds the light spots where the light beams 401 aand 401 b impinge on a nominally aligned scale grating 430. Generally,the ability of various encoder reader configurations according to thisinvention to minimize errors related to pitch misalignment and dynamicpitch misalignment is improved as the dimension of the first zone alongthe measuring axis direction 101 is reduced. Therefore, the dimension ofthe first zone along the measuring axis direction 101 generally providesthe proper balance between reducing errors related to pitch misalignmentand dynamic pitch misalignment and achieving the other design objectivesof a particular embodiment according to this invention. In variousexemplary embodiments, the dimension of the first zone along themeasuring axis direction 101 is longer, to provide clearance betweenvarious diffracted and reflected light beams and various elements of theencoder readhead, as discussed below with reference to FIGS. 22A-22C,for example. In various other exemplary embodiments, the dimension ofthe first zone along the measuring axis direction 101 is equal to orless than 4 times the nominal spot length along the measuring axisdirection 101. Exemplary nominal spot lengths are discussed below withreference to FIG. 14. It should be appreciated that the sensitivity tostatic and dynamic pitch misalignments is primarily related to thedistance between the spots in the first zone 431. Thus, it should beappreciated that due to design choices or various misalignments thelight beams 401 a and 401 b may actually diverge for a small distancebefore impinging on the scale grating 430 and after converging proximateto the first zone, provided that the light beams 401 a and 401 bnevertheless impinge on a nominally aligned scale grating 430 in a firstzone 431 according to the principles of this invention.

[0090] The split light beams 401 a and 401 b enter the first zone 431,giving rise to respective diffracted light beams 402 a and 402 b,respectively, which are respective diffraction orders diffracted alongrespective light paths which diverge proximate to the first zone 431.The retroreflectors 440 and 441 receive the diffracted light beams 402 aand 402 b, respectively, along their respective light paths. It shouldbe appreciated that if the light beams 401 a and 401 b actually divergefor a small distance before impinging on the scale grating 430 and afterconverging proximate to the first zone, then in a complementary way thediffracted light beams 402 a and 402 b beams may actually converge for asmall distance after leaving the scale grating 430 and before divergingproximate to the first zone, provided that the diffracted light beams402 a and 402 b beams nevertheless diverge proximate to the first zonealong respective optical paths to be received by the retroreflectors 440and 441 according to the principles of this invention.

[0091] The retroreflectors 440 and 441 receive the diffracted lightbeams 402 a and 402 b, respectively, along their respective light pathsand reflect them as light beams 402 ar and 402 br, respectively. Thus,the light paths of the light beams 402 ar and 402 br are parallel to thelight paths of the diffracted light beams 402 a and 402 b according tothe principles of this invention. As a result, according to theprinciples of this invention, the exemplary encoder readheadconfiguration 400 is substantially insensitive to errors related to yawmisalignment and dynamic yaw misalignment. The retroreflectors 440 and441 shown in FIG. 4 are represented by exemplary corner-cube typeretroreflectors. However, a cats-eye type retroreflector or any othernow known or later developed type of retroreflector may be used,provided that it is operable according to the principles of thisinvention.

[0092] In various exemplary embodiments, depending on the location ofthe retroreflectors 440 and 441 relative to the diffracted light beams402 a and 402 b, respectively, the light paths of the light beams 402 arand 402 br may be offset, that is, separated from the diffracted lightbeams 402 a and 402 b in a direction transverse to the measuring axisdirection 101 as shown in FIG. 4, or along the measuring axis direction101, or both. Such an offset prevents light from reentering the lightsource and causing light source instability in various embodimentsaccording to this invention, and also helps to eliminate “crossoverbeams” in various other embodiments according to this invention. Thus,these problems present in the prior art configurations previouslydescribed with reference to FIGS. 1 and 2 may be avoided in theexemplary encoder readhead configuration 400, and in various otherembodiments according to this invention which include such a beamseparation offset due to the placement of retroreflectors 440 and 441relative to the diffracted light beams 402 a and 402 b, respectively. Itshould be appreciated that such embodiments according to this inventionneed not include polarizers, thus avoiding unwanted encoder readheadfabrication and/or assembly constraints in some situations. However, itshould be further appreciated that polarizers may be optionally includedin such embodiments, as desired for compatibility with a particularoptical detector 460 in a particular embodiment, for example.

[0093] In any case, the light beams 402 ar and 402 br are retroreflectedalong light paths parallel to the light paths of the diffracted lightbeams 402 a and 402 b, thus converging proximate to the second zone 432in a manner similar to the convergence of the light beams 401 a and 401b proximate to the first zone 431.

[0094] The second zone 432, similar to the first zone 431, is a nominalzone on the nominal plane of the scale grating 430 that has a dimensionalong the measuring axis direction 101 that bounds the light spots wherethe light beams 402 ar and 402 br impinge on a nominally aligned scalegrating 430. The design considerations related to the second zone 432and the ability of various encoder reader configurations according tothis invention to minimize errors related to pitch misalignment anddynamic pitch misalignment are the same as those previously describedwith reference to the first zone 431. The dimension of the second zone432 along the measuring axis direction 101 is determined similarly tothe dimension of the first zone 431. As previously discussed, in variousexemplary embodiments, depending on the location of the retroreflectors440 and 441 relative to the diffracted light beams 402 a and 402 b,respectively, the light paths of the light beams 402 ar and 402 br maybe offset, that is, separated from the diffracted light beams 402 a and402 b in a direction transverse to the measuring axis direction 101 asshown in FIG. 4. In such a case the second zone is dimensioned similarlyto the first zone 431, and is offset from the first zone 431 in adirection transverse to the measuring axis direction 101 as shown inFIG. 4, corresponding to the offset of the light paths of the lightbeams 402 ar and 402 br. More generally, in various exemplaryembodiments the second zone 432 is nominally offset from the first zone431 in the same direction as any offset of the light paths of the lightbeams 402 ar and 402 br relative to the diffracted light beams 402 a and402 b. Furthermore, in various embodiments according to this inventionthe light paths of the light beams 402 ar and 402 br may fully orpartially overlap the light paths of the diffracted light beams 402 aand 402 b, and in such cases the second zone 432 similarly may fully orpartially overlap the first zone 431.

[0095] The retroreflected light beams 402 ar and 402 br enter the secondzone 432, giving rise to respective later-diffracted light beams 403 aand 403 b, respectively, which are respective diffraction ordersdiffracted along respective light paths which diverge proximate to thesecond zone 432 in a manner similar to the divergence of the light beams402 a and 402 b proximate to the first zone 431. The later-diffractedlight beams 403 a and 403 b are then directed by the light beamdirecting elements 420 and 421 such that the light beamslater-diffracted light beams 403 a and 403 b converge proximate to thebeam splitting element 415.

[0096] It should be appreciated that in the embodiment shown in FIG. 4,and various other embodiments shown herein, the light beam directingelements 420 and 421 are represented as exemplary plane mirrors and thelater-diffracted light beams 403 a and 403 b each impinge upon and arereflected once from a second portion of the same plane mirrors having afirst portion impinged upon by the split light beams 401 a and 401 b,respectively. However, as previously discussed, in various embodimentsaccording to this invention the light paths of the light beams 402 arand 402 br may fully or partially overlap the light paths of thediffracted light beams 402 a and 402 b, and in such cases the lightpaths of the later-diffracted light beams 403 a and 403 b may fully orpartially overlap the light paths of the split light beams 401 a and 401b, respectively. In such cases, previously discussed second portions andfirst portions of each plane mirror may similarly fully or partiallyoverlap. More generally, each of the light beam directing elements 420and 421 impinged upon by the later-diffracted light beams 403 a and 403b may be any now known or later developed light beam directing elementor combination of elements, such as suitably arranged mirrors and/orportions of a grating or multiple gratings or the like, that areoperable through one or more reflections or deflections to direct thelater-diffracted light beams 403 a and 403 b to converge proximate tothe beam splitting element 415.

[0097] The beam splitting element 415 receives the later-diffractedlight beams 403 a and 403 b and reflects or deflects at least one of thebeams according to known optical principles to bring thelater-diffracted light beams 403 a and 403 b into alignment or nearalignment in the shared zone 450, as shown in FIG. 4. The aligned ornearly aligned later-diffracted light beams 403 a and 403 b then enter asuitably-chosen optical detector 460.

[0098] As illustrated in FIG. 4, and various other figures shown herein,the beam splitting element 415 is conceptually represented as a readheadgrating element and the later-diffracted light beams 403 a and 403 beach impinge upon a second portion of the same readhead grating elementwhich has a first portion impinged upon by the light beam 401.Furthermore, in the illustration in FIG. 4, the later-diffracted lightbeams 403 a and 403 b beams are shown partially aligned in the sharedzone 450. The structure and operation of exemplary readhead gratingelements which are consistent with this illustration are discussed belowwith reference to FIG. 10B or 10C. However, in this and various otherexemplary embodiments herein, the beam splitting element 415 impingedupon by the later-diffracted light beams 403 a and 403 b is moregenerally intended to represent any now known or later developed lightbeam splitting element or combination of elements, such as a suitablyarranged “half-silvered” beamsplitter or a polarizing beamsplitter orportions thereof, or portions of a grating or multiple gratings or thelike, including any of the configurations discussed below and shown in10A-10E or 11A-11D, which is operable to produce split light beamsaccording to the principles of this invention.

[0099] Various alternative ways of bringing the later-diffracted lightbeams 403 a and 403 b into alignment or near alignment in the sharedzone 450, as well as the operation of respective suitably-chosen opticaldetectors, will be apparent to one skilled in the art. Applicabledescriptions of various beam alignment and detection techniques are alsoincluded in the discussion of FIGS. 1 and 2 above and in theincorporated '895, '085 and '833 patents, as well as in relateddiscussion further below. It should be appreciated that the aligned orpartially aligned beams in the shared zone 450 will give rise to anillumination characteristic in the shared zone which varies periodicallyin correspondence with the relative displacement between the encoderreadhead and the scale grating 430. Because each of the aligned orpartially aligned beams in the shared zone 450 have been twicediffracted by the scale 430, once before entering the reflectingelements 440 or 441 and once after, when the grating scale 430 moves byone pitch increment the illumination characteristic in the shared zonewill undergo 4 periodic cycles.

[0100] In various embodiments according to this invention, certainportions of the light from the later-diffracted light beams 403 a and403 b may be lost due to the operation of the beam splitting element415. Such lost light is symbolically indicated by the lost light 499shown in FIG. 4. However, other than possible disturbance of thestability of the light source 412 as discussed herein, such lost lightis not significant to the operation of this invention. As previouslydiscussed, in general, lost light is not discussed herein, unless it isrelevant to light source disturbance or crossover light in a particularembodiment.

[0101] In the embodiment shown in FIG. 4, and various other embodimentsshown herein, the exemplary beam paths associated with the beams 401 a,401 b, 403 a and 403 b are inclined in a respective first direction awayfrom a plane aligned parallel to the measuring axis direction 101 andnormal to the nominal plane of the scale grating 430. The exemplary beampaths associated with the beams 402 a, 402 b, 402 ar and 402 br areinclined in an opposite direction away from the plane aligned parallelto the measuring axis direction 101 and normal to the nominal plane ofthe scale grating 430. However, it should be appreciated that in variousother embodiments according to the principles of this invention, theseinclinations can be greater or lesser than indicated in the variousexemplary embodiments shown herein and still prevent undesirable mixingor crossover of various light beams, while at the same time facilitatingcompact design and packaging of the device.

[0102]FIG. 5A is a three-dimensional conceptual view of an opticaldisplacement sensing device configuration in accordance with a secondexemplary embodiment of the invention. FIGS. 5B, 5C and 5D are,respectively, a side view, a top view and an end view of the opticaldisplacement sensing device shown in FIG. 5A. In FIGS. 5A-5D, evidentlycorresponding elements and/or elements having similar reference numbersare arranged and operate as previously described with reference to FIG.4 unless otherwise indicated. The generic discussions with reference toFIG. 4 apply in a corresponding manner to FIGS. 5A-5D, unless otherwiseindicated. Furthermore, in FIGS. 5A-5D the reference numbers of severalelements evidently corresponding to previously discussed generic orspecific elements are omitted, since their arrangement and operationhave already been made evident and are not further described withreference to FIGS. 5A-5D. Conversely, in FIGS. 5A-5D the referencenumbers of elements are generally repeated or added if there is arelated description or if the FIGS. 5A-5D serve to further clarify suchelements.

[0103] The embodiment shown in FIGS. 5A-5D is a practical configurationthat is substantially insensitive to various misalignments and dynamicmisalignments, yet offers substantial flexibility to locating andfabricating and assembling various optical components in a compact andeconomical way in an encoder readhead. FIG. 5A emphasizes the splitlight beam input portion 410 including a laser diode light source 412Aand a beamsplitter 415X, the scale grating 430 having a grating pitch P,the shared zone 450, and an optical detector assembly 460A (shownsymbolically) having one or more power and signal connections 461. Invarious exemplary embodiments herein the grating pitch P is chosen to be0.4 μm. However, in various other exemplary embodiments the pitch P maybe chosen in a range from less than 0.4 μm up to many microns.

[0104] The laser diode light source 412A receives power, emits coherentradiation, and in various exemplary embodiments includes an integratedcollimating element and emits collimated light. In various exemplaryembodiments the laser diode light source 412A is chosen to emit lighthaving a wavelength of approximately 635 nm and in various otherembodiments to emit light having a wavelength of 405 nm, but anyconvenient wavelength may be used. In various exemplary embodiments, avertical cavity surface emitting laser diode is used for the laser diodelight source 412A. The beamsplitter element 415X receives the light beam401 from the laser diode light source 412A and produces two split lightbeams 401 a and 401 b according to known optical principles.

[0105] It should be appreciated that in the embodiment shown in FIGS.5A-5D, and in various other embodiments shown herein, the exemplarybeamsplitter 415X is a beamsplitter such as one of the beamsplittersdescribed below with reference to FIG. 11C or 11D having a first portionimpinged upon by the light beam 401, or any now known or later developedpolarizing beamsplitter which provides similar functions. However, invarious other exemplary embodiments, the beamsplitter 415X impinged uponby the light beam 401 may be any now known or later developedbeamsplitter element or combination of elements, such as a suitablyarranged “half-silvered” beamsplitter or a polarizing beamsplitter orportions thereof, or the like, including any of the configurationsdiscussed below and shown in FIGS. 11A-11D, which is operable to producesplit light beams according to the principles of this invention, andwhich is also usable in conjunction with the particular optical detectortype chosen for a particular embodiment.

[0106] The beamsplitter element 415X inputs the two split light beams401 a and 401 b to the remainder of the encoder readhead 500, aspreviously described. FIGS. 5B-5D further clarify one exemplary pathconfiguration for the previously described series of light beams 401,401 a, 401 b, 402 a, 402 b, 402 ar, 402 br, 403 a, and 403 b.

[0107] As best shown in FIG. 5B, the later-diffracted light beams 403 aand 403 b converge proximate to the beamsplitter 415X. The beamsplitter415X receives the later-diffracted light beams 403 a and 403 b, andreflects and transmits them, respectively, into alignment in the sharedzone 450. The beamsplitter 415X further operates such that the reflectedand transmitted light beams aligned in the shared zone are orthogonallypolarized relative to each other. The aligned orthogonally polarizedbeams then enter the optical detector assembly 460A.

[0108] It should be appreciated that in the embodiment shown in FIGS.5A-5D, and in various other embodiments shown herein, thelater-diffracted light beams 403 a and 403 b each impinge upon a secondportion of the same exemplary beamsplitter 415X which has a firstportion impinged upon by the light beam 401. However, in various otherexemplary embodiments, the beamsplitter/combiner impinged upon by thelater-diffracted light beams 403 a and 403 b may be any now known orlater developed light beam splitting/combining element or combination ofelements, such as a suitably arranged “half-silvered” beamsplitter or apolarizing beamsplitter or portions thereof, or the like, including anyof the configurations discussed below and shown in FIGS. 11A-11D, whichis operable in conjunction with the particular type of optical detectorselected for a particular embodiment.

[0109] The aligned orthogonally polarized beams then enter the opticaldetector assembly 460A, which in the embodiment shown in FIGS. 5A-5D isarranged and operates similarly to the optical detector 460P describedbelow with reference to FIG. 21. The optical detector assembly 460A thenoutputs one or more signals on the one or more power and signalconnections 461, the signals usable to determine the displacement of thescale grating 430 relative to the encoder readhead. More generally, theoptical detector assembly 460A can be any now known or later developedoptical detector which provides signals that are usable to determine therelative phase between the lights of the orthogonally polarized beamsentering the optical detector. Various alternative detector schemes willbe apparent to one skilled in the art. Instructive descriptions ofdetector schemes are also included in the discussion of FIGS. 1 and 2above and in the incorporated '895, '085 and '833 patents.

[0110] As previously described, in various embodiments shown herein, theexemplary beam paths associated with the beams 401 a, 401 b, 403 a and403 b are inclined in a respective first direction away from a planealigned parallel to the measuring axis direction 101 and normal to thenominal plane of the scale grating 430, and the exemplary beam pathsassociated with the beams 402 a, 402 b, 402 ar and 402 br are inclinedin an opposite direction away from the plane aligned parallel to themeasuring axis direction 101 and normal to the nominal plane of thescale grating 430. For purposes of clarification, FIG. 5D shows anexemplary normal plane 475 aligned parallel to the measuring axisdirection 101 and normal to the nominal plane of the scale grating 430,an exemplary inclination plane 474 inclined in a respective firstdirection away from the normal plane 475, and the inclination angle 473between them. The inclination angle 473 is also referred to as the angle“delta” or “δ” herein. It should appreciated that the inclination angle473 can be greater or lesser than indicated in the exemplary embodimentshown in FIG. 5D and still prevent undesirable mixing or crossover ofvarious light beams, while at the same time facilitating compact designand packaging of the device.

[0111] Also shown in FIG. 5D is a nominal beam splitting heightdimension 471, corresponding to the height of the beam splitting portionof any beam splitting element, such as the beamsplitter 415X, above thenominal plane of the scale grating 430. Also shown in FIG. 5D is anominal retroreflector height dimension 472, corresponding to themaximum height of any reflective surface of any retroreflector 440 or441 above the nominal plane of the scale grating 430. It has been foundthat in embodiments according to this invention where the beam splittingelement is a beamsplitter such as the beamsplitter 415X, or any of thebeamsplitters shown in FIGS. 11A-11D, or the like, that the errors insuch embodiments related to pitch misalignment and dynamic pitchmisalignment and the like tend to be relatively reduced when theretroreflector height dimension 472 is made smaller. It has also beenfound that in embodiments according to this invention where the beamsplitting element is a readhead grating such as the readhead gratingsshown in FIGS. 10A-10E, or the like, that the errors in such embodimentsrelated to pitch misalignment and dynamic pitch misalignment and thelike tend to be relatively reduced as the beam splitting heightdimension 471 is made smaller and/or as the retroreflector heightdimension 472 approaches equality with the beam splitting heightdimension 471.

[0112] It should be appreciated that the planes 474 and 475, the angle473 and the heights 471 and 472 are intended to be genericallyillustrative of similarly numbered or evidently analogous planes, anglesand heights in various other embodiments, thus, their locations andvalues are not limited by the exemplary embodiment shown in FIG. 5D.More generally, it should be appreciated that any new genericdiscussions included in the preceding discussion of FIGS. 5A-5D areindicative of a number of configurations which vary in their combinationof elements and dimensions from the specific elements and dimensionsindicated in FIGS. 5A-5D. Thus, FIGS. 5A-5D should not be interpreted aslimiting with regard to the meaning and intent of the generic portionsof the preceding discussion.

[0113]FIG. 6 is a three-dimensional schematic view clarifyingconventional geometric components and symbols used for describing lightpath directions herein. FIG. 6 shows a set of orthogonal X, Y and Zaxes. The X axis is aligned parallel to the measuring axis direction101. The Z axis is aligned normal to the grating surface of a nominallyaligned scale grating, and the Y axis is aligned orthogonal to the X andZ axes at the grating surface of a nominally aligned scale grating.Three angles are used to indicate the orthogonal components of unitvector 601, each of which are measured from a respective principal axisas shown in FIG. 6. The angle formed between the unit vector 601 and thex axis is α, the angle formed between the unit vector 601 and the y axisis β, and the angle formed between the unit vector 601 and the z axis isγ. The respective x axis, y axis and z axis components of the unitvector 601 are thus, cos α, cos β and cos γ, respectively, as shown inFIG. 6. This same terminology is useful for describing the orientationand relationship of various light beam paths in various exemplaryembodiments according to this invention.

[0114]FIG. 7 is a three-dimensional schematic view showing a genericinput light ray direction and the resulting conical distribution ofdiffracted light rays from a grating analogous to the scale grating 430described herein, using the terminology of FIG. 6. In FIG. 7 an inputlight ray 701 impinges on grating 702 with input angles (α, β, γ) andvarious diffracted output beams each having a respective set of outputangles (α₁, β₁, γ₁) result. The various diffracted output beams, shownas arrows in FIG. 7, together define a cone 703. The relationshipbetween the input angles for a beam incident on the grating 702, and theoutput angles for the nth diffraction order are as follows:Input  Workspace  Date:09/18/2003  Number:10101031  Folder:03

[0115] where n is the diffraction order, λ is the wavelength of thelight, and d is the grating pitch. The grating pitch is also referred toas P herein. One skilled in the art will readily understand that theseequations may be used to determine an operable arrangement of encoderreadhead components for any embodiment of an encoder readheadconfiguration according to the principles of this invention, includingconfigurations where the inclination angle 473, best shown in FIG. 5D,is not equal to zero.

[0116]FIG. 8 shows an error table 801, which lists as entries the errormagnitudes associated with various dynamic misalignments and drifts fora prior art optical displacement sensing device configurationcorresponding to FIG. 1. The individual dynamic misalignments and driftslisted across the tops of the columns occur in various combinations withthe individual initial misaligmnents or deviations from nominal (alsoreferred to as initial “static” misalignments or deviations, herein)listed down the left end of the rows. “Gap” refers to the separationbetween an encoder readhead and a scale grating surface along adirection normal to the nominal scale grating surface. A scale gratingsurface in its intended nominal design plane in an encoder readheadconfiguration according to the principles of this invention has a gapmisalignment of zero.

[0117] For all entries in table 801, the nominal wavelength λ is 0.635μm, the nominal angle α is 38 degrees, the nominal angle β is 80degrees, and the grating period of the scale is 0.4 μm. Additionally,the results are for an encoder readhead configuration approximately asshown in FIG. 1, where the element corresponding to element 102 shown inFIG. 1 is a readhead grating positioned at a beam splitting heightdimension of 10 mm relative the to the scale grating surface and havinga grating pitch of 0.8 μm. Each entry in the table corresponds to thedisplacement measurement error, in nanometers, that arises when theindividual initial installation misalignment listed at the left of theentry row is combined with the individual dynamic misalignments listedat the top of the entry column. Each dynamic misalignment represents achange in alignment that occurs between the time a reference positionmeasurement is made, and the time the displacement measurement is made.Accordingly, each entry is the measurement error that is included in theapparent displacement indicated by the encoder readhead signals, betweenthe reference position and the displacement position, due to the dynamicmisalignment or deviation.

[0118] As a clarifying example, in table 801, given an installationpitch misalignment of 30 minutes, the error associated with either adynamic roll, or yaw misalignment of 2 seconds is 0.0 nm. For the sameinstallation pitch misalignment, a dynamic wavelength drift of 0.00025μm produces an error of 1.0 nm magnitude and a dynamic gap change of0.01 mm produces an error of 87.3 nm. It should be appreciated that this87.3 nm error is the result of an “apparent” geometric translation ofthe encoder readhead relative to the scale grating along the measuringaxis, and is thus an error that will appear for a wide variety ofencoder readhead configurations, regardless of their design. Moreimportantly, for purposes of comparison with various encoder readheadembodiments according to this invention, the significant aspect of table801 is that a dynamic pitch of only 2 seconds of arc in this prior artconfiguration produces 97.0 nm displacement measuring error for allinitial alignment conditions. As previously discussed, dynamic pitchmisalignment is one of the most prevalent and/or difficult to eliminateconditions in many practical encoder applications, and is thus ofparticular significance for encoder readhead design. As previouslydiscussed, dynamic pitch errors are substantially reduced by the priorart configurations shown in FIGS. 2 and 3, but those configurations havethe other limitations previously discussed.

[0119]FIG. 9 shows an error table 901, which lists as entries the errormagnitudes associated with various dynamic misalignments and drifts fora optical displacement sensing device configuration according to thisinvention which corresponds to the basic configuration shown in FIGS.5A-5D, and more closely to basic configuration described with referenceto FIGS. 12A-12D below, having the beamsplitter 415X positioned at abeam splitting height dimension 471 of 10 mm and the retroreflectors 440and 441 positioned at retroreflector height dimension 472 of 5 mm. Forall entries in table 901, the nominal wavelength λ is 0.635 μm, thenominal angle α is 38 degrees, the nominal angle β is 80 degrees, andthe grating period of the scale is 0.4 μm. The significant aspect oftable 901 is that the underlying encoder readhead embodiment accordingto this invention virtually eliminates dynamic pitch error. It should befurther appreciated that the underlying encoder readhead embodimentaccording to this invention achieves this performance while at the sametime overcoming the various previously discussed limitations of theprior art configurations shown in FIGS. 2 and 3. It has been determinedthat various other encoder readhead embodiments according to thisinvention, including but not limited to those corresponding to FIGS. 4,5A-5D, 12A-12D, 19, 20A-20C, 22A-22C, 23 and 24 and their discussedvariations, also substantially reduce dynamic pitch errors whileovercoming at least one of the previously discussed limitations of theprior art configurations shown in FIGS. 2 and 3.

[0120] The most significant errors shown in table 901 are for dynamicwavelength deviation and dynamic gap change. The errors associated withdynamic gap change have the same magnitude in table 901 and table 801,and have been previously discussed. The error associated with dynamicwavelength deviation may be overcome by using more stable light sourcesand/or improved temperature control in various exemplary embodiments.However, it has also been determined that various encoder readheadconfigurations according to the principles of this invention alsoinfluence the magnitude of the errors associated with dynamic wavelengthdeviation, which is therefore discussed further with reference to FIGS.13-18 below.

[0121] FIGS. 10A-10E are three-dimensional views clarifying theoperation of respective first through fifth readhead gratingconfigurations usable in the beam splitting portion of various exemplaryencoder readhead embodiments according to this invention. Each viewshows hypothetical apertures on the surfaces of the readhead gratings.These apertures are not physical elements, but are illustrated only toclarify the paths of the light beams in the figures.

[0122]FIG. 10A shows readhead grating element 415A having aconfiguration wherein the input beam 401 impinges on a first portion 416having a first grating pitch to produce the split beams 401 a and 401 bby transmission of a zero order beam and by transmission of a first orhigher order diffracted beam. The paths of the beams 401, 401 a, 401 b,403 a and 403 b shown in FIGS. 10A-10E have been generally shown anddescribed previously, with reference to FIGS. 4 and 5A-5D. Thelater-diffracted light beams 403 a and 403 b impinge on a second portion417 having the first grating pitch, which transmits the beam 403 b as azero order beam and the beam 403 a as a first or higher order diffractedbeam, to bring the beams 403 a and 403 b into alignment in the sharedzone 450, as shown. In various exemplary embodiments, the first readheadgrating pitch is the same as the pitch of the scale grating used withthat embodiment. The readhead grating element 415A may be used inencoder readhead embodiments which prevent “crossover beams” aspreviously described, that is, where lights arising from the split beams401 a and 401 b do not share a common path before reaching the sharedzone 450. However, the resulting illumination characteristic in theshared zone may not, by itself, indicate the direction of the relativedisplacement and thus has limited application. This problem may beremedied by polarizing the encoder readhead beams in the manner describebelow with reference to FIG. 10D or 10E, but inserting the polarizersinto the beam path remotely from the readhead grating element, and usinga polarization sensitive detector such as the optical detector describedwith reference to FIG. 21 below.

[0123]FIG. 10B shows readhead grating element 415B having aconfiguration which operates identically to readhead grating element415A, except that the second portion 417 has a second grating pitchvarying from the first grating pitch of the first portion 416. Thus, thebeam 403 a is diffracted as a first or higher order diffracted beam thatis slightly out of alignment with the transmitted beam 403 b in theshared zone 450, which gives rise to an interference pattern in theshared zone 450, represented conceptually by the interference pattern451. The interference pattern 451 translates spatially in correspondenceto the scale grating displacement. The translation of the interferencepattern 451 can be quantitatively detected by various optical detectorarray techniques known in the art and/or currently commerciallyavailable, including the use of quadrature arrays which directly producequadrature signals, or arrays which allow the interference patterntranslation to be imaged and digitally analyzed at a higher resolution.

[0124]FIG. 10C shows readhead grating element 415C having aconfiguration which produces the same result as readhead grating element415B. In the readhead grating element 415C, the first portion 416 andthe second portion 417 have the same grating pitch, but an optical wedgeelement 491 is added. Thus, the beam 403 b is transmitted as zero orderbeam that is slightly out of alignment with the diffracted order beam403 a in the shared zone 450, which gives rise to an interferencepattern in the shared zone 450, represented conceptually by theinterference pattern 451.

[0125] The readhead grating elements 415B or 415C have configurationsthat may be used in any encoder readhead embodiments which prevent“crossover beams” as previously described, that is, where lights arisingfrom the split beams 401 a and 401 b do not share a common path beforereaching the shared zone 450.

[0126]FIG. 10D shows a readhead grating element 415D having aconfiguration which produces the same result as readhead grating element415A, except the beams aligned in the shared zone are, in addition,mutually orthogonally polarized. In the readhead grating element 415D,the first portion 416 and the second portion 417 have the same gratingpitch, but a first polarizer 492 and a second polarizer 493 are arrangedto insure that the beams 403 a and 403 b are mutually orthogonallypolarized before entering the shared zone 450. The readhead gratingelement 415A may be used in any encoder readhead embodiments whichprevent “crossover beams” as previously described, that is, where lightsarising from the split beams 401 a and 401 b do not share a common pathbefore reaching the polarizers 493 and 492, respectively. Theillumination characteristic in the shared zone 450 may be sensed using apolarization sensitive detector such as the optical detector describedwith reference to FIG. 21 below.

[0127]FIG. 10E shows a readhead grating element 415E having aconfiguration which produces the same result as readhead grating element415A, except the split beams 401 a and 401 b are, in addition, mutuallyorthogonally polarized. In the readhead grating element 415E, the firstportion 416 and the second portion 417 have the same grating pitch. Afirst polarizer 494 and a second polarizer 495 are arranged to insurethat the beams 401 a and 401 b are mutually orthogonally polarizedbefore any of the resulting beams “cross over” or share a common beampath. Furthermore, after any beams “cross over” or share a common beampath in the encoder readhead, the first polarizer 494 and the secondpolarizer 495 are also arranged to filter the beams 403 a and 403 b suchthat they are mutually orthogonally polarized in the same orientation asthe beams 401 a and 401 b, respectively. Because of the initialpolarization of the split beams 401 a and 401 b, any crossover beamsand/or beams sharing common beam paths can be removed by this finalfiltering of the beams 403 a and 403 b. Thus, the configuration ofreadhead grating element 415E may be used in any encoder readheadembodiments according to this invention, including configurations whichdo not include the beam offsets, as previously described, and/or whichdo not include an appreciable inclination angle, as previouslydescribed. The illumination characteristic in the shared zone 450 may besensed using a polarization sensitive detector such as the opticaldetector described with reference to FIG. 21 below.

[0128] FIGS. 11A-11D are three-dimensional views clarifying theoperation of respective first through fourth beamsplitter configurationsusable in the beam splitting portion of various exemplary encoderreadhead embodiments according to this invention. Each view showshypothetical apertures on the surfaces of the beamsplitters. Theseapertures are not physical elements, but are illustrated only to clarifythe paths of the light beams in the figures. The beamsplitterconfigurations shown in FIGS. 11A-11D are described by analogy withcorresponding readhead grating configurations shown in the FIGS. 10A and10C-10E. However, in each FIGS. 11A-11D, at the first portion 416 andthe second portion 417, the beams 401 b and 403 a, respectively, arereflected at a partially reflective beamsplitter interface 418 arrangedvertically, rather than diffracted at a horizontal grating surface as inthe FIGS. 10A and 10C-10E. Each beamsplitter configuration has the sameresult in the shared zone 450 as the indicated analogous readheadgrating configuration, and may be similarly used with similar encoderreadhead embodiments according to this invention. Their operation willbe apparent to one skilled in the art.

[0129]FIG. 1A shows beamsplitter element 415F which has a configurationanalogous to the configuration of readhead grating element 415A shown inFIG. 10A. FIG. 1B shows beamsplitter element 415G which has an opticalwedge element 491 added in a configuration analogous to theconfiguration of readhead grating element 415C shown in FIG. 10C. FIG.11C shows beamsplitter element 415H which has a first polarizer 492 anda second polarizer 493 arranged to insure that the beams 403 a and 403 bare mutually orthogonally polarized before entering the shared zone 450in a configuration analogous to the configuration of readhead gratingelement 415D shown in FIG. 10D. FIG. 11D shows a beamsplitter element415J which has a first polarizer 494 and a second polarizer 495 arearranged in a configuration analogous to the configuration of readheadgrating element 415E shown in FIG. 10D to insure that the beams 401 aand 401 b are mutually orthogonally polarized before any of theresulting beams “cross over” or share a common beam path, and that afterany beams “cross over” or share a common beam path in the encoderreadhead, the first polarizer 494 and the second polarizer 495 filterthe beams 403 a and 403 b such that they are mutually orthogonallypolarized in the same orientation as the beams 401 a and 401 b,respectively.

[0130] It should be appreciated for all of the readhead gratingconfigurations shown in FIGS. 10A-10E and 11A-11D, the operable portionsof each element are the portions struck by the light beams. Thus, invarious exemplary embodiments, these portions may be more separated,less separated, provided as integrated assemblies or as separateelements, or the like, so long as they are positioned with respect tooperable encoder readhead beam paths in accordance with the principlesof this invention.

[0131]FIG. 12A is a three-dimensional conceptual view of an opticaldisplacement sensing device in accordance with a third exemplaryembodiment of the invention. FIGS. 12B, 12C and 12D are, respectively, aside view, a top view and an end view of the optical displacementsensing device in shown in FIG. 12A. The embodiment shown in FIGS.12A-12D is analogous to the embodiment shown in FIGS. 5A-5D, except forthe location of the retroreflectors 440 and 441. The generic discussionswith reference to FIGS. 5A-5D apply in a corresponding manner to FIGS.12A-12D, unless otherwise indicated. Thus, in FIGS. 12A-12D thereference numbers of several elements evidently corresponding topreviously discussed generic or specific elements are omitted, sincetheir arrangement and operation have already been made evident.Conversely, in FIGS. 12A-12D the reference numbers of elements aregenerally repeated or added if there is a related description or if theFIGS. 12A-12D serve to further clarify such elements.

[0132] The embodiment shown in FIGS. 12A-12D is a practicalconfiguration that is substantially insensitive to various misalignmentsand dynamic misalignments, yet exhibits substantial flexibility forfabricating, locating and assembling various optical components in acompact and economical way in an encoder readhead. FIG. 12A emphasizesthe angle a formed between the split light beams 401 a and 401 b thatimpinge on the scale grating 430 in the first zone 431 and ahypothetical line 1201 which extends parallel to the measuring axisdirection 101 through the respective points of impingement of the lightbeams 401 a and 401 b. The angle a shown in FIG. 12A corresponds to theangle α defined with reference for FIG. 6. Thus, the angle α lies in theplane 474, best seen in FIG. 12D. Various errors related to dynamicmisalignment or drift are discussed further below in relation to variousdesign values for the angle a.

[0133] FIGS. 12A-12D also emphasize a configuration and/or height forthe retroreflectors 440 and 441 that results in relatively short opticalpath lengths for the light beams 402 a, 402 ar, 402 b and 402 br,respectively. As previously discussed, it has been found that inembodiments according to this invention where the beam splitting elementis a beamsplitter, such as any of the beamsplitters shown in FIGS.11A-11D, or the like, the errors in such embodiments related to pitchmisalignment and dynamic pitch misalignment and the like tend to bereduced as the retroreflector height dimension 472 is decreased.Furthermore, shorter optical path lengths tend to reduce the sensitivityof the encoder readhead to dynamic variations in the wavelength of thelight source. It also generally permits the encoder readhead to be mademore compact. The beam splitting height dimension 471 and retroreflectorheight dimension 472 for this exemplary configuration are best seen inFIGS. 12B and 12D.

[0134]FIG. 12B also shows portions of an exemplary readhead componenthousing 1225 and a gap dimension 1220 between the bottom of the readheadcomponent housing 1225 and the surface of the scale grating 430. Theportions of the readhead component housing 1225 are not shown in FIGS.12A, 12C and 12D so that the optical paths may be seen more clearly. Itshould be appreciated that factors limiting how close the light beamdirecting elements 420 and 421, and the retroreflectors 440 and 441 canbe placed to the scale, in order to shorten the related optical paths,are the desired operating gap dimension 1220 and practical wallthicknesses and mounting arrangements within the readhead componenthousing 1225. In various exemplary embodiments, to achieve short opticalpath lengths and a compact encoder readhead, the bottom edges of lightbeam directing elements 420 and 421, and/or the retroreflectors 440 and441 are operatively positioned proximate to the bottom of the readheadcomponent housing 1225 with due consideration to any design constraintsimposed by optical path clearance considerations and the desiredoperating gap dimension 1220. Considering that a smaller gap dimensionshortens various optical paths to reduce errors and a larger gapdimension simplifies installation, in various exemplary embodiments thedesired gap dimension 1220 may be on the order of 1-2 mm, for example.

[0135] As shown in FIGS. 12A-12D, the light beam paths 402 a, 402 ar,402 b and 402 br are offset from the light beam paths 401 a, 403 a, 401b and 403 b, respectively, by both a vertical separation angle component1221, best seen in FIG. 12B, and by the inclination angle 473, “delta”,best seen in FIG. 12D. It should be appreciated that the verticalseparation angle component 1221 and the inclination angle 473 may bechosen in combination to reduce the height and/or the width of theassociated encoder readhead, as emphasized by the hypothetical dimensionbox 1230 shown in FIG. 6D. It can be seen that if the inclination angle473 is decreased, to maintain light beam clearance between the lightbeam directing elements 420, 421 and the retroreflectors 440, 441 theheight of the dimension box 630 must increase, corresponding toincreased encoder readhead height. Similarly, if the vertical separationangle component 1221 is decreased, to maintain light beam clearance thewidth of the dimension box 630 must increase, corresponding to increasedencoder readhead width. It should be appreciated that the planes 474 and475, the angles 1221 and 473 and the heights 471 and 472 are intended tobe generically illustrative of analogous planes, angles and heights invarious other embodiments, thus, their locations and values are notlimited by the exemplary embodiment shown in FIGS. 12A-12D, but are tobe chosen to achieve the overall encoder readhead dimension desired in aparticular application. Furthermore, FIGS. 12A-12D are indicative of anumber of additional embodiments according to this invention which varyin their combination of elements and dimensions from the specificelements and dimensions indicated. Thus, FIGS. 12A-12D should not beinterpreted as limiting with regard to the teachings discussed above.

[0136] FIGS. 13-18 show information useful for determining desirabledesign dimensions for the angle a, previously discussed with referenceto FIGS. 6, 7 and 12A, and for the inclination angle “delta”, previouslydiscussed with reference to FIGS. 5D and 12D.

[0137]FIG. 13 is a graph showing error sensitivity data for dynamic gapmisalignment and wavelength change at various incident beam angles α inan optical displacement sensing device in accordance with this inventionwhere the scale grating pitch is 0.4 μm, the light source has a nominal635 nm wavelength, a beam splitter is positioned at a height 10 mm abovethe scale grating surface, the retroreflectors are positioned such thatthe path length from the scale to the corner of the retroreflectors is10 mm, the angle α lies in a plane of incidence which is perpendicularto the scale grating grooves and to the scale (that is, the angle deltais 0°), and the static pitch misalignment is set at 0.5°. A curve 1301shows the dynamic errors resulting at various values of the angle α whenthe light source wavelength dynamically changes by 0.25 nm. A curve 1302shows the dynamic errors resulting at various values of the angle α whenthere is a dynamic gap change of 10 μm. As previously discussed withreference to FIG. 9, these tend to be the largest remaining dynamicerrors in various exemplary embodiments according to this invention,therefore, it is especially useful to choose design values which furtherreduce these dynamic errors.

[0138] With the stated combination of wavelength and scale gratingpitch, the associated encoder readhead is operable when α ranges fromapproximately 50 degrees to 10 degrees. However, as shown by the curves1301 and 1302, for the stated combination of readhead parameters, theerrors associated with dynamic wavelength change and gap change increaserapidly when α is greater than approximately 40 degrees. Therefore, invarious exemplary embodiments according to this invention, thecomponents of the encoder readhead are configured such that α is lessthan or equal to 40 degrees.

[0139]FIG. 14 shows additional design considerations for selecting adesign value for the angle α. FIG. 14 assumes a beam having a nominaldiameter of 1.0 mm and a wavelength of 635 nm is directed onto a scalegrating having a grating pitch of 0.4 μm from a height 5.0 mm above thesurface of the scale grating, at various values of the angle α. Thesecond column of the table shown in FIG. 14 shows data comparing theresulting maximum beam cross section dimension at the retroreflectorsversus the angle a shown in the first column. The third column of thetable shown in FIG. 14 shows data comparing the distance between mirrorsused as beam directing elements at the height 5.0 mm above the surfaceof the scale grating versus the angle α shown in the first column. Theresulting maximum beam cross section dimension varies from 4.6 mm whenthe angle α is 10 degrees to 0.9 mm when the angle α is 40 degrees. Theresulting distance between beam directing mirrors varies from 56.7 mmwhen in the angle α is 10 degrees to 11.9 mm when in the angle α is 40degrees.

[0140] One design consideration in various exemplary embodimentsaccording to this invention is that the spot on a retroreflector becomeselongated and overfills the aperture of the retroreflection element,thereby wasting light, and/or falling on imperfect edges of theretroreflector. Another design consideration is that the distancebetween beam directing mirrors may determine the overall length of theencoder readhead. A beam cross section of 4.6 mm, requires relativelylarger retroreflector elements, leading to a relatively larger encoderreadhead. A distance between mirrors of 56.7 mm likewise leads to arelatively larger encoder readhead. Therefore, even though lesser anglesa generally reduce the sensitivity to various dynamic errors as shown inFIG. 13, in various exemplary embodiments, the angle α is made greaterthan or equal to 20 degrees, to enable a smaller overall size for theencoder readhead.

[0141]FIG. 15 is a graph showing error sensitivity data for dynamic gapmisalignment at various incident beam angles a and at variousinclination angles (delta), for the encoder parameters previouslydescribed with reference to FIG. 13, unless otherwise indicated. Thedynamic gap change is 10 μm. The curves 1501-1503, corresponding toinclination angles of delta=0 degrees, delta=15 degrees and delta=45degrees, respectively, are nearly indistinguishable. However, as bestshown by the curves 1501 and 1502, for the stated combination ofreadhead parameters, the errors associated with dynamic gap changeincrease more rapidly when α is greater than approximately 40 degrees.Therefore, in various exemplary embodiments according to this invention,the components of the encoder readhead are configured such that α isless than or equal to 40 degrees.

[0142]FIG. 16 is a graph identical to the graph shown in FIG. 15, exceptthat the light source wavelength in nominally 405 nm, instead of 635 nm.The curves 1601-1603, corresponding to inclination angles of delta=0degrees, delta=15 degrees and delta-45 degrees, respectively, areindistinguishable. Compared to the results shown in FIG. 15, the reducedlight source wavelength reduces the dynamic errors due to gap change,for the stated combination of readhead parameters. Furthermore, theerrors associated with dynamic gap change increase relatively slowly upto an operating range of 60 degrees for the angle α. Therefore, invarious exemplary embodiments according to this invention, thecomponents of the encoder readhead are configured such that α is lessthan or equal to 60 degrees. However, since the errors associated withdynamic gap change are restricted to values below approximately 50 nmwhen the angle α is less than or equal to 45 degrees and the wavelengthand grating pitch are as stated, in various exemplary embodimentsaccording to this invention, the components of the encoder readhead areconfigured such that α is less than or equal to 45 degrees.

[0143]FIG. 17 is a graph showing error sensitivity data for dynamicwavelength drift at various incident beam angles and at various inclinedplane angles, for the encoder parameters previously described withreference to FIG. 13, unless otherwise indicated. The curves 1701-1703correspond to inclination angles of delta=0 degrees, delta=15 degreesand delta=45 degrees, respectively. The light source wavelengthdynamically changes by 0.25 nm. This dynamic light source wavelengthchange produces significantly larger dynamic errors than those due tothe due to gap change previously discussed with reference to FIGS. 15and 16. Thus, if the light source of an encoder readhead exhibits thistype of instability, for high accuracy it is particularly important tochoose an angle a that reduces this sensitivity. For the curves 1701 and1702, the errors associated with dynamic wavelength change increaserelatively slowly up to an operating range of 40 degrees for the angleα. Therefore, in various exemplary embodiments according to thisinvention, the components of the encoder readhead are configured suchthat α is less than or equal to 40 degrees. However, for the curve 1703,corresponding the angle delta=45 degrees, the errors increase morerapidly versus the angel α. Therefore, in various other exemplaryembodiments according to this invention, the components of the encoderreadhead are configured such that α is less than or equal to 30 degrees.

[0144]FIG. 18 is a graph identical to the graph shown in FIG. 17, exceptthat the light source wavelength in nominally 405 nm, instead of 635 nm.The curves 1801-1803, corresponding to inclination angles of delta=0degrees, delta=15 degrees and delta=45 degrees, respectively, are nearlyindistinguishable. Compared to the results shown in FIG. 17, the reducedlight source wavelength reduces the dynamic errors due to wavelengthchange, for the stated combination of readhead parameters. Furthermore,the errors associated with dynamic wavelength change increase relativelyslowly up to an operating range of 60 degrees for the angle α.Therefore, in various exemplary embodiments according to this invention,the components of the encoder readhead are configured such that α isless than or equal to 60 degrees. However, since the errors associatedwith dynamic wavelength change are restricted to values belowapproximately 100 nm when the angle that a is less than or equal to 45degrees and the wavelength and grating pitch are as stated, in variousexemplary embodiments according to this invention, the components of theencoder readhead are configured such that α is less than or equal to 45degrees.

[0145]FIG. 19 is a three-dimensional conceptual view of an opticaldisplacement sensing device in accordance with a fourth exemplaryembodiment within the scope of the invention. The configuration shown inFIG. 19 is analogous to the configuration shown in FIGS. 5A-5D, exceptthe inclination angle 473 shown in FIG. 5D is chosen to be zero in theconfiguration shown in FIG. 19. Because the inclination angle is zero,the embodiment shown in FIG. 19 is a practical configuration that offersthe possibility to fabricate a relatively narrow encoder readhead.However, when the inclination angle is zero, the zero-order reflectionbeams arising from the light beams 401 a, 401 b, 402 a and 402 b in thefirst zone 431, and from the light beams 402 ar, 402 br, 403 a and 403 bin the second zone 432, become cross-over beams, as previouslydiscussed. Thus, for the configuration shown in FIG. 19, the beamsplitting element 415 of the split light beam input portion 410 shouldincorporate polarizing elements to separate or filter out the crossoverbeams, such as the readhead grating shown in FIG. 10E, or thebeamsplitter shown in FIG. 11D, or the like. Furthermore, the opticaldetector assembly 460 should include further include polarizingelements, such as the optical detector 460P described below withreference to FIG. 21, or the like. More generally, the optical detectorassembly 460 can be any now known or later developed optical detectorwhich provides signals that are usable to determine the relative phasebetween the lights of orthogonally polarized beams entering the opticaldetector. Various alternative detector schemes will be apparent to oneskilled in the art. Instructive descriptions of detector schemes arealso included in the discussion of FIGS. 1 and 2 above and in theincorporated '895, '085 and '833 patents.

[0146]FIG. 20A is a conceptual side view of an optical displacementsensing device in accordance with a fifth exemplary embodiment withinthe scope of the invention. FIG. 20B is a top view of the opticaldisplacement sensing device shown in FIG. 20A and FIG. 20C is an endview of the optical displacement sensing device shown in FIG. 20A. Theconfiguration shown in FIGS. 20A-20C is analogous to the configurationshown in FIGS. 12A-12D, except the components of the encoder readheadare arranged such that the portion of each of the light beams thatimpinges upon or is diffracted from the scale grating 430, as best shownby the numbered light beam portions shown in FIG. 20B, forms the sameangle with respect to a hypothetical line which extends parallel to themeasuring axis direction 101 and through the respective points ofimpingement or diffraction. Such a configuration can be designed inaccordance with EQUATIONS 1-3, previously discussed.

[0147] In such a configuration, using a suitable inclination angle thelight beam directing elements 420, 421 and the retroreflectors 440, 441may be positioned at approximately the same height, as best shown inFIG. 20C, which in various exemplary embodiments results in a readheadhaving a relatively small overall height. However, because each of thelight beams that impinges upon or is diffracted from the scale grating430 forms the same angle with respect to the hypothetical line whichextends parallel to the measuring axis direction 101, the zero-orderreflection beams arising from the light beams 401 a, 401 b, 402 a and402 b in the first zone 431, and from the light beams 402 ar, 402 br,403 a and 403 b in the second zone 432, become cross-over beams, aspreviously discussed. Thus, for the configuration shown in FIGS.20A-20C, the beam splitting element 415 of the split light beam inputportion 410 should incorporate polarizing elements to separate or filterout the crossover beams, such as the readhead grating shown in FIG. 10E,or the beamsplitter shown in FIG. 11D, or the like. Furthermore, theoptical detector assembly 460 should include polarizing elements, suchas the optical detector 460P described below with reference to FIG. 21,or the like, as previously discussed with reference to FIG. 19.

[0148]FIG. 21 is a schematic side view of an optical detector 460Pincluding a polarizer configuration. The optical detector 460P is, thus,useful in various exemplary embodiments according to the invention thatinclude crossover beams. More generally, the optical detector 460P isuseful in various exemplary embodiments wherein the light beams 403 aand 403 b are orthogonally polarized in the shared zone 450 for anyreason. In operation, orthogonally polarized light beams 403 a and 403 bare emitted from a beam splitting element 415 into the shared zone 450along aligned optical paths to optical detector 460P. Beam splitter 4606splits the orthogonally polarized light beams 403 a and 403 b into twosplit detection beams. A first split detection beam passes through aquarter wave plate 4653 and a polarizer 4671 to a photodetector element4681. A second split detection beam passes through a polarizer 4672 to aphotodetector element 4682. The polarizers 4671 and 4672 are eachnominally oriented at a 45 degree angle between the orthogonallypolarized beams that reach them, and are further arranged to passmutually orthogonally polarized light. The quarter wave plate 4653delays one of the output light beams a quarter of a wave or 90 degrees.Hence, detector 4681 detects a signal that is 90 degrees phase shiftedrelative to the signal detected by detector 4682, providing a well knownquadrature signal format on one or more signal lines 461. In thisembodiment of the invention, the optical detector 460P is shown as oneintegrated unit. However, other configurations of a detector may be usedto achieve the same objective as the configuration shown in FIG. 21, aswill be apparent to one skilled in the art.

[0149]FIG. 22A is a conceptual side view of an optical displacementsensing device in accordance with a sixth exemplary embodiment withinthe scope of the invention. FIG. 22B is a top view of the opticaldisplacement sensing device shown in FIG. 22A, and FIG. 22C is an endview of the optical displacement sensing device in shown in FIG. 22A.The configuration shown in FIGS. 22A-22C is analogous to theconfiguration shown in FIGS. 20A-20C. The components of the encoderreadhead are arranged such that the portion of each of the light beamsthat impinges upon or is diffracted from the scale grating 430 forms thesame angle with respect to a hypothetical line which extends parallel tothe measuring axis direction 101 and through the respective points ofimpingement or diffraction. However, in contrast to the configurationshown in FIGS. 20A-20C, the impingement points of the light beams in thefirst zone 431 and the second zone 432, respectively, are separatedalong the measuring axis direction, as best shown in FIG. 22B. As aresult, the zero order reflection paths represented by the lines 2201and 2202 shown in FIG. 22A, fall outside the effective apertures of thelight beam directing elements 420, 421 and the retroreflectors 440, 441,eliminating potential crossover beams. Thus, the configuration shown inFIGS. 22A-22C need not include polarizers in the light beam paths or inthe optical detector. However, similarly to the configuration shown inFIGS. 20A-20C, in such a configuration the light beam directing elements420, 421 and the retroreflectors 440, 441 may be positioned atapproximately the same height, which in various exemplary embodimentsresults in a readhead having a relatively small overall height.

[0150]FIG. 23 is a conceptual side view of an optical displacementsensing device in accordance with a seventh exemplary embodiment withinthe scope of the invention. In the configuration shown in FIG. 23, theoptical readhead components are positioned such that the light beampaths 402 a, 402 ar, 402 b and 402 br lie nearer to the scale gratingthan the light beam paths 401 a, 403 a, 401 b and 403 b. Thisconfiguration may be used with an inclination angle which eliminatescrossovers beams, and thus eliminates the need for polarizers in theconfiguration. Alternatively, for inclination angles which do noteliminate crossover beams, the beam splitting element 415 of the splitlight beam portion 410 may include polarizers to separate out thecrossover beams, as previously discussed.

[0151]FIG. 24 is a conceptual side view of an optical displacementsensing device in accordance with an eighth exemplary embodiment withinthe scope of the invention. In the configuration shown in FIG. 24, thescale grating 430 is a transmission grating and the retroreflectors 440,441 are positioned to receive transmitted diffracted orders. If theconfiguration is arranged such that transmitted zero-order beams falloutside the effective apertures of the light beam directing elements420, 421, and the retroreflectors 440, 441, as represented by the lines2401-2404, then crossover beams will be eliminated from theconfiguration, thus eliminating the need for polarizers in theconfiguration, regardless of the inclination angle used. Alternatively,for configurations which do not eliminate crossover beams, the beamsplitting element 415 of the split light beam portion 410 may includepolarizers to separate out the crossover beams, as previously discussed.

[0152]FIG. 25 is a three-dimensional view showing a first exemplarytransparent block configuration usable in various exemplary embodimentsaccording to the invention. Refractive effects on the various light beampaths are ignored in FIG. 25, but the adjustments needed for sucheffects will be readily understood by one skilled in the art. FIG. 25shows a transparent block 2500 including a first block portion 2510 anda second block portion 2515 which are joined at an interface which isfabricated to include a half-silvered mirror or the like in order toprovide a beam splitting element 415. The first block portion 2510includes an end surface 2502 and orthogonal retroreflector surfaces 2504and 2506 that are all coated to be reflective. The second block portion2515 includes an end surface 2503 and orthogonal retroreflector surfaces2505 and 2507 that are all coated to be reflective. The transparentblock 2500 is usable for providing the beam path configuration andencoder readhead operating characteristics previously described withreference to FIG. 19. Thus, the transparent block 2500 must also providea polarizing function equivalent to the beamsplitter shown in FIG. 1D,or the like, in order to separate out crossover beams, as previouslydescribed with reference to FIG. 19. In various exemplary embodiments,this is achieved by incorporating mutually orthogonal polarizers on theportions of the end surfaces 2502 and 2503 where the beams 401 a, 403 a,401 b and 403 b, are reflected.

[0153] The transparent block 2500 is usable to provide a very compactand dimensionally stable encoder readhead assembly. It should beappreciated that the transparent block 2500 may be fabricated by bondingtogether, or fabricating together, various combinations of individualoptical elements. The optical pieces may comprise a fewer number of morecomplex individual optical elements or a greater number of less complexindividual optical elements. The individual elements may be joined alongany combination of bonding planes (not shown) which allow practical,compact and accurate fabrication of the optical pieces. The opticalpieces may further incorporate mounting provisions for being joined tovarious light sources, photodetector arrangements and/or encoderreadhead mounting elements. Thus, the configuration shown in FIG. 25 isan exemplary configuration only, and is not intended to be limiting.

[0154]FIG. 26A is a three-dimensional view showing a second exemplarytransparent block configuration usable in various exemplary embodimentsaccording to the invention. FIGS. 26B and 26C are side and end views,respectively, clarifying the beam paths provided by the transparentblock configuration shown in FIG. 26A. Refractive effects on the variouslight beam paths are included in all figures. The nominal light beamdiameters are reduced only in FIG. 26A, to better distinguish thevarious beam paths. FIG. 26A shows a light source 412, including acollimating lens 413, which inputs a light beam 401 to a transparentblock 2600. An optical detector 460 is also shown. The transparent block2600 includes a left half block portion 2510 and a right half blockportion 2515. An upper left half portion 2612 and an upper right halfportion 2617 are joined at an interface which is fabricated to include ahalf-silvered mirror or the like in order to provide a beam splittingelement 415. The left half block portion 2610 includes an end surface2602 and the left half of orthogonal retroreflector surfaces 2604 and2605, which are all coated to be reflective, and which together comprisethe retroreflector 440, as best shown in FIGS. 26B and 26C. The righthalf block portion 2615 includes an end surface 2603 and the right halfof orthogonal retroreflector surfaces 2604 and 2605, which are allcoated to be reflective, and which together comprise the retroreflector441, as best shown in FIGS. 26B and 26C. The transparent block 2600 isusable for providing the beam path configuration and encoder readheadoperating characteristics previously described with reference to FIGS.5A-5D or 12A-12D, for example, when the nominal retroreflector heightdimension 472 is suitably chosen. The transparent block 2600 providesfor suitable non-zero inclination angles 473. Thus, in contrast to theblock configuration shown in FIG. 25, crossover beams may be eliminatedand polarizers are not needed.

[0155] In various exemplary embodiments, the transparent block 2600incorporates an optical deflection element (not shown), which providesthe same function as the optical wedge element 491 previously discussedwith reference FIG. 11B. The optical deflection element is incorporatedon the portion of the end surface 2602 or 2603 where the beam 403 a or403 b, respectively, is reflected, solely to enable the use of anoptical detector scheme which uses an array detector to detect aninterference fringe pattern such as the interference fringe pattern 451discussed with reference to FIG. 1B. The resulting interference patterntranslates spatially in correspondence to the scale gratingdisplacement. The translation of the interference pattern can bequantitatively detected by various optical detector array techniquesknown in the art and/or currently commercially available. In variousalternative embodiments the optical detector 460 used with thetransparent block 2600 includes the use of a quadrature array whichdirectly produces quadrature signals, and in various other embodimentsthe optical detector 460 includes the use of a linear or 2-dimensionsCCD arrays or the like, allows the interference pattern translation tobe imaged and digitally analyzed at a higher resolution. Any of thesearrays may be assembled to the transparent block 2600 at a desiredposition, orientation and spacing, by means of adhesives for example, toprovide a precise, stable, economical and compact encoder readhead.

[0156] In various other exemplary embodiments, mutually orthogonalpolarizers are incorporated on the portions of the end surfaces 2602 and2603 where the beams 403 a and 403 b and/or 401 a and 401 b, arereflected, solely to enable the use of an optical detector scheme whichuses polarizers.

[0157] The transparent block 2600 is usable to provide a very compactand dimensionally stable encoder readhead assembly. It should beappreciated that the transparent block 2600 may be fabricated by bondingtogether, or fabricating together, various combinations of individualoptical elements. It should further be appreciated that although thebeams 402 a and/or 402 b appear to impinge on corner of the surfaces2602 and 2604, and 2603 and 2604, respectively, before impinging on thesurface 2605 to complete the retroreflection, in various exemplaryembodiments the beam 402 a avoids the corner and impinges on thesurfaces 2602 and 2604 in succession. Likewise, the beam 402 b avoidsthe corner and impinges on the surfaces 2603 and 2604 in succession, inorder to avoid wavefront aberrations induced by imperfect corners. Thus,the configuration shown in FIGS. 26A-26C is an exemplary configurationonly, and is not intended to be limiting.

[0158]FIG. 27A is a three-dimensional view showing a third exemplarytransparent block configuration usable in various exemplary embodimentsaccording to the invention. FIGS. 27B and 27C are side and end views,respectively, clarifying the beam paths provided by the transparentblock configuration shown in FIG. 27A. The transparent blockconfiguration shown in FIGS. 27A-27C is usable for providing the beampath configuration and encoder readhead operating characteristicspreviously described with reference to FIG. 4, for example. Refractiveeffects on the various light beam paths are included in all figures. Thenominal light beam diameters are reduced only in FIG. 27A, to betterdistinguish the various beam paths.

[0159]FIG. 27A shows a light source 412, including an input beamdirecting element 414, which inputs a light beam 401 to a transparentblock 2700. An optical detector 460 is also shown. The transparent block2700 includes a left half block portion 2710 and a right half blockportion 2715. The left half block portion 2710 includes a rear endsurface 2702, a forward end surface 2704 and the left half of orthogonalretroreflector surfaces 2706 and 2707, which are all coated to bereflective. The forward end surface 2704 and the left half of orthogonalretroreflector surfaces 2706 and 2707 together comprise theretroreflector 440, as best shown in FIGS. 27B and 27C. The right halfblock portion 2715 includes a rear end surface 2703, a forward endsurface 2705 and the right half of orthogonal retroreflector surfaces2706 and 2707, which are all coated to be reflective. The forward endsurface 2705 and the right half of orthogonal retroreflector surfaces2706 and 2707 together comprise the retroreflector 441, as best shown inFIGS. 27B and 27C. A top rear surface of the transparent block 2700includes a transmissive readhead grating, which in various embodimentsmay be any of the readhead gratings previously discussed with referenceto FIGS. 10A-10E, and which may be fabricated as a separate element orintegrated to the surface in order to provide a beam splitting element415. As previously discussed, the inventor has found that in embodimentsaccording to this invention where the beam splitting element is areadhead grating such as the readhead gratings shown in FIGS. 10A-10E,or the like, that the errors in such embodiments related to pitchmisalignment and dynamic pitch misalignment and the like tend to berelatively reduced as the beam splitting height dimension 471 is madesmaller, and/or the retroreflector height dimension 472 approachesequality with the beam splitting height dimension 471, as provided bythe exemplary transparent block 2700.

[0160] The transparent block 2700 provides for non-zero inclinationangles 473. Thus, similar to the block configuration shown in FIGS.26A-26C, crossover beams may be eliminated and polarizers are notneeded. In various exemplary embodiments, the transparent block 2700incorporates an optical deflection element (not shown), which providesthe same function as the optical wedge element 491 previously discussedwith reference FIG. 10C. The optical deflection element is incorporatedon the portion of the rear end surface 2702 or 2703 where the beam 403 aor 403 b, respectively, is reflected, solely to enable the use of anoptical detector scheme which uses an array detector to detect aninterference fringe pattern, as previously discussed with reference toFIGS. 26A-26C. Alternatively, the readhead grating shown in FIG. 10B maybe used, to achieve the same effect. In such cases, the optical detector460 includes an array detector according to previously describedinterference fringe detection schemes, and may be assembled to thetransparent block 2600 at a desired position, orientation and spacing,by means of adhesives for example, to provide a precise, stable,economical and compact encoder readhead.

[0161] In various other exemplary embodiments, mutually orthogonalpolarizers are incorporated on the portions of the rear end surfaces2702 and 2703 where the beams 403 a and 403 b and/or 401 a and 401 b,are reflected, solely to enable the use of an optical detector schemewhich uses polarizers.

[0162] The transparent block 2700 is usable to provide a very compactand dimensionally stable encoder readhead assembly. It should beappreciated that the transparent block 2700 may be fabricated by bondingtogether, or fabricating together, various combinations of individualoptical elements. It should further be appreciated that although thebeams 402 a and/or 402 b appear to impinge on corner of the surfaces2704 and 2706, and 2705 and 2706, respectively, before impinging on thesurface 2707 to complete the retroreflection, in various exemplaryembodiments the beam 402 a avoids the corner and impinges on thesurfaces 2704 and 2706 in succession. Likewise, the beam 402 b avoidsthe corner and impinges on the surfaces 2705 and 2706 in succession, inorder to avoid wavefront aberrations induced by imperfect corners. Thus,the configuration shown in FIGS. 26A-26C is an exemplary configurationonly, and is not limiting.

[0163]FIG. 28 shows a flow diagram for a method of using an opticaldisplacement sensing device in accordance with various exemplaryembodiments of the invention. The method begins at a block S2810, byinputting split light beams to be received by respective light directingelements of the optical displacement sensing device. The method thencontinues to a block S2820, by directing the split light beams to afirst zone on a scale grating using various light directing elements. Ina preferred embodiment, the split light beams area directed alongnominally symmetrical paths to the first zone. The method then continuesto a block S2830.

[0164] At block S2830, the split light beams are each diffracted fromthe first zone on the scale grating, and two diffracted beams aredirected along divergent paths and enter respective retroreflectors. Themethod then continues to a block S2840, by retroreflecting the beamsentering the respective retroreflectors back to a second zone on thescale grating. The method then continues to a block S2850, where thebeams retroreflected back to the second zone on the scale are eachdiffracted from the second zone on the scale grating, and two diffractedbeams are directed along divergent paths after leaving the second zone.The method then continues to a block S2860.

[0165] At block S2860, the two diffracted beams along divergent pathsafter leaving the second zone are directed along respective paths to ashared zone, using various light directing elements. In a preferredembodiment, the beams directed to the shared zone are directed alongnominally symmetrical paths. The method then continues to a block S2870,where the method ends by detecting at least one illuminationcharacteristic arising from the shared zone and determining a relativedisplacement of the scale grating relative to the optical displacementsensing device based on the detection result.

[0166] While this invention has been described in conjunction with thespecific embodiments above, it should be appreciated that these specificembodiments offer many alternatives, combinations, modifications, andvariations. As one example, although the various embodiments accordingto the invention are shown herein as linear transducers, the designs maybe used in or adapted to cylindrical and circular rotary applications byone of ordinary skill in the art. As a separate example, this inventionmay use light wavelengths outside the visible spectrum, provided thatsuch wavelengths are operable with scale gratings and optical detectorsaccording to the principles of this invention. Accordingly, thepreferred embodiments of this invention, as set forth above, areintended to be illustrative and not limiting. Various changes can bemade without departing from the spirit and scope of this invention.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A device for measuringthe relative displacement of a scale along a measuring axis, the scalehaving a grating formed along the measuring axis, the device comprising:a split light beam input portion for inputting two split light beamsalong respective light paths; two or more light beam directing elementsfor directing the two split light beams along respective light pathswhich converge proximate to a first zone on the scale grating to giverise to two diffracted light beams along respective light paths whichdiverge proximate to the first zone; two or more retroreflector elementsfor receiving the two diffracted light beams along respective lightpaths and retroreflecting the two diffracted light beams alongrespective light paths which converge proximate to a second zone on thescale grating to give rise to two later-diffracted light beams alongrespective light paths which diverge proximate to the second zone; andan optical detector; wherein the two later-diffracted light beams alongrespective light paths which diverge proximate to the second zone aredirected to enter a shared zone and the optical detector detects atleast one illumination characteristic arising from the shared zone, thedetected at least one illumination characteristic usable to determinethe relative displacement of the scale.
 2. The device of claim 1,wherein the optical detector is usable to output at least one outputsignal which is indicative of the at least one detected illuminationcharacteristic usable to determine the relative displacement of thescale.
 3. The device of claim 1, wherein the respective light pathswhich diverge proximate to the first zone each receive a diffractedlight beam arising from only one of the split beams.
 4. The device ofclaim 1, wherein the respective light paths which diverge proximate tothe second zone each receive a later-diffracted light beam arising fromonly one of the retroreflected light beams.
 5. The device of claim 1,wherein the scale grating has a pitch, the pitch of the scale gratingbeing less than a wavelength of light input by the split light beaminput portion.
 6. The device of claim 1, wherein the respective lightpaths which converge proximate to the first zone on the scale grating atleast partially overlap at a nominal positioning plane for the scalegrating.
 7. The device of claim 1, wherein the respective light pathswhich converge proximate to the first zone on the scale gratingnominally fully overlap at a nominal positioning plane for the scalegrating.
 8. The device of claim 1, wherein the respective light pathswhich converge proximate to the second zone on the scale grating atleast partially overlap at a nominal positioning plane for the scalegrating.
 9. The device of claim 1, wherein the respective light pathswhich converge proximate to the second zone on the scale gratingnominally fully overlap at a nominal positioning plane for the scalegrating.
 10. The device of claim 1, wherein the first and second zonesat least partially overlap, and wherein the respective light paths whichconverge proximate to the second zone and the respective light pathswhich converge proximate to the first zone at least partially overlap ata nominal positioning plane for the scale grating.
 11. The device ofclaim 1, wherein the first and second zones are separated from oneanother along a direction perpendicular to the measuring axis.
 12. Thedevice of claim 1, wherein the two split light beams nominally impingeon the first zone on the scale grating such that each forms the samerespective angle less than or equal to 60 degrees and greater than orequal to 10 degrees relative to a line extending parallel to themeasuring axis and through their respective impingement points.
 13. Thedevice of claim 12, wherein the respective angle is less than or equalto 60 degrees and greater than or equal to 20 degrees.
 14. The device ofclaim 12, wherein the respective angle is less than or equal to 50degrees and greater than or equal to 20 degrees.
 15. The device of claim12, wherein the respective angle is less than or equal to 40 degrees andgreater than or equal to 20 degrees.
 16. The device of claim 12, whereinthe respective angle is less than or equal to 30 degrees and greaterthan or equal to 20 degrees.
 17. The device of claim 12, wherein thescale grating has a pitch, the pitch of the scale grating being lessthan a wavelength of the light input by the split light beam inputportion.
 18. The device of claim 17, wherein the pitch of the scalegrating is less than 0.8 times the wavelength of the light and therespective angle is less than or equal to 45 degrees and greater than orequal to 20 degrees.
 19. The device of claim 1, wherein at least aportion of the respective light paths which converge proximate to thefirst zone nominally coincide with a plane aligned parallel to themeasuring axis and normal to a nominal positioning plane for the scalegrating, and at least a portion of the respective light paths whichdiverge proximate to the first zone nominally coincide with the planealigned parallel to the measuring axis and normal to the nominalpositioning plane for the scale grating.
 20. The device of claim 1,wherein at least a portion of the respective light paths which convergeproximate to the first zone are inclined in a first direction away froma plane aligned parallel to the measuring axis and normal to a nominalpositioning plane for the scale grating, and at least a portion of therespective light paths which diverge proximate to the first zone areinclined in an opposite direction away from the plane aligned parallelto the measuring axis and normal to the nominal positioning plane forthe scale grating.
 21. The device of claim 1, wherein the measuring axiscomprises a circular track and the scale grating is formed along atleast a portion of the circular track, the scale is operable byrotation, and the device is usable to measure the angular displacementof the scale.
 22. The device of claim 1, wherein the two or more lightbeam directing elements comprise at least one plane mirror surface foreach respective split light beam path.
 23. The device of claim 22,wherein a first portion of each respective plane mirror surface is usedfor directing a respective split light beam and a second portion of eachrespective plane mirror surface is used for directing a respectivelater-diffracted light beam.
 24. The device of claim 22, wherein the twoor more light beam directing elements comprise opposing surfaces of ablock of transparent material.
 25. The device of claim 1, wherein thetwo or more retroreflector elements comprise respective corner regionsof a block of transparent material.
 26. The device of claim 1, whereinthe split light beam input portion comprises a light source and one of abeam splitter and a grating.
 27. The device of claim 26, wherein thelight source comprises one of a laser diode and laser light receivedthrough an optical fiber.
 28. The device of claim 26, wherein the splitlight beam input portion comprises the beam splitter, and the two splitlight beams are input at a first height relative to a nominalpositioning plane for the scale grating, and for each of the two or moreretroreflector elements the reflective portion which is farthest fromnominal positioning plane for the scale grating is positioned at aheight which is not more than 80% of the first height.
 29. The device ofclaim 26, wherein the split light beam input portion comprises thegrating, and the two split light beams are input at a first heightrelative to a nominal positioning plane for the scale grating, and foreach of the two or more retroreflector elements the reflective portionwhich is farthest from nominal positioning plane for the scale gratingis positioned at a height which is more than 80% and less than 120% ofthe first height.
 30. The device of claim 1, wherein the opticaldetector comprises an optical array and the at least one illuminationcharacteristic arising from the shared zone arises from an interferencefringe pattern in the shared zone.
 31. The device of claim 1, whereinthe device further comprises at least one polarizing element arrangedsuch that the two later-diffracted light beams along respective lightpaths which diverge proximate to the second zone are orthogonallypolarized upon entering the shared zone; and the optical detectorcomprises two or more polarization sensitive detector portions.
 32. Amethod for determining the relative displacement of a scale along ameasuring axis, the scale having a diffraction grating formed along themeasuring axis, the method comprising: transmitting a light beam from alight source onto a light beam splitting element to generate two splitlight beams; directing the two split light beams along respective lightpaths which converge proximate to a first zone on the scale grating;diffracting the two split light beams in the first zone to produce twodiffracted light beams along respective light paths which divergeproximate to the first zone and which enter respective retroreflectors;retroreflecting the two diffracted light beams along respective lightpaths which converge proximate to a second zone on the scale grating;diffracting the two diffracted light beams to produce twolater-diffracted light beams along respective light paths which divergeproximate to the second zone and which enter a shared zone; anddetecting at least one illumination characteristic arising from theshared zone, the detected at least one illumination characteristicusable to determine the relative displacement of the scale.
 33. Themethod of claim 32, further comprising outputting at least one outputsignal from the optical detector which is indicative of the at least onedetected illumination characteristic; and determining the relativedisplacement of the scale based on the at least one output signal. 34.The method of claim 32, wherein the respective light paths which divergeproximate to the first zone each receive a diffracted light beam arisingfrom only one of the split beams.
 35. The method of claim 32, whereinthe respective light paths which diverge proximate to the second zoneeach receive a later-diffracted light beam arising from only one of theretroreflected light beams.
 36. The method of claim 32, wherein thescale grating has a pitch, the pitch of the scale grating being lessthan a wavelength of the light transmitted from the light source. 37.The method of claim 32, wherein the respective light paths whichconverge proximate to the first zone on the scale grating at leastpartially overlap at a nominal positioning plane for the scale grating.38. The method of claim 32, wherein the respective light paths whichconverge proximate to the first zone on the scale grating nominallyfully overlap at a nominal positioning plane for the scale grating. 39.The method of claim 32, wherein the respective light paths whichconverge proximate to the second zone on the scale grating at leastpartially overlap at a nominal positioning plane for the scale grating.40. The method of claim 32, wherein the respective light paths whichconverge proximate to the second zone on the scale grating nominallyfully overlap at a nominal positioning plane for the scale grating. 41.The method of claim 32, wherein the first and second zones at leastpartially overlap, and wherein the respective light paths which convergeproximate to the second zone and the respective light paths whichconverge proximate to the first zone at least partially overlap at anominal positioning plane for the scale grating.
 42. The method of claim32, wherein the first and second zones are separated from one anotheralong a direction perpendicular to the measuring axis.
 43. The method ofclaim 32, wherein the two split light beams nominally impinge on thefirst zone on the scale grating such that each forms the same respectiveangle less than or equal to 60 degrees and greater than or equal to 10degrees relative to a line extending parallel to the measuring axis andthrough their respective impingement points.
 44. The method of claim 43,wherein the respective angle is less than or equal to 60 degrees andgreater than or equal to 20 degrees.
 45. The method of claim 43, whereinthe respective angle is less than or equal to 50 degrees and greaterthan or equal to 20 degrees.
 46. The method of claim 43, wherein therespective angle is less than or equal to 40 degrees and greater than orequal to 20 degrees.
 47. The method of claim 43, wherein the respectiveangle is less than or equal to 30 degrees and greater than or equal to20 degrees.
 48. The method of claim 43, wherein the scale grating has apitch, the pitch of the scale grating being less than a wavelength ofthe light transmitted from the light source.
 49. The method of claim 48,wherein the scale grating has a pitch, the pitch of the scale gratingbeing less than 0.8 times the wavelength of the light transmitted fromthe light source and the respective angle is less than or equal to 45degrees and greater than or equal to 20 degrees.
 50. The method of claim32, wherein at least a portion of the respective light paths whichconverge proximate to the first zone are inclined in a first directionaway from a plane aligned parallel to the measuring axis and normal to anominal positioning plane for the scale grating, and at least a portionof the respective light paths which diverge proximate to the first zoneare inclined in an opposite direction away from the plane alignedparallel to the measuring axis and normal to the nominal positioningplane for the scale grating.